path: root/src/nix-env/nix-env.cc

#include "profiles.hh"
#include "names.hh"
#include "globals.hh"
#include "misc.hh"
#include "shared.hh"
#include "parser.hh"
#include "eval.hh"
#include "help.txt.hh"
#include "nixexpr-ast.hh"
#include "get-drvs.hh"
#include "attr-path.hh"
#include "pathlocks.hh"
#include "common-opts.hh"
#include "xml-writer.hh"
#include "store-api.hh"
#include "db.hh"
#include "util.hh"

#include <cerrno>
#include <ctime>
#include <algorithm>
#include <iostream>
#include <sstream>

#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>


using namespace nix;
using std::cout;


typedef enum {
    srcNixExprDrvs,
    srcNixExprs,
    srcStorePaths,
    srcProfile,
    srcAttrPath,
    srcUnknown
} InstallSourceType;


struct InstallSourceInfo
{
    InstallSourceType type;
    Path nixExprPath; /* for srcNixExprDrvs, srcNixExprs */
    Path profile; /* for srcProfile */
    string systemFilter; /* for srcNixExprDrvs */
    ATermMap autoArgs;
    InstallSourceInfo() : autoArgs() { };
};


struct Globals
{
    InstallSourceInfo instSource;
    Path profile;
    EvalState state;
    bool dryRun;
    bool preserveInstalled;
    bool keepDerivations;
    string forceName;
};


typedef void (* Operation) (Globals & globals,
    Strings opFlags, Strings opArgs);


void printHelp()
{
    cout << string((char *) helpText, sizeof helpText);
}


static bool isNixExpr(const Path & path)
{
    struct stat st;
    if (stat(path.c_str(), &st) == -1)
        throw SysError(format("getting information about `%1%'") % path);

    return !S_ISDIR(st.st_mode) || pathExists(path + "/default.nix");
}


static void getAllExprs(EvalState & state,
    const Path & path, ATermMap & attrs)
{
    Strings names = readDirectory(path);

    for (Strings::iterator i = names.begin(); i != names.end(); ++i) {
        Path path2 = path + "/" + *i;
        if (isNixExpr(path2))
            attrs.set(toATerm(*i), makeAttrRHS(
                    parseExprFromFile(state, absPath(path2)), makeNoPos()));
        else
            getAllExprs(state, path2, attrs);
    }
}


static Expr loadSourceExpr(EvalState & state, const Path & path)
{
    if (isNixExpr(path)) return parseExprFromFile(state, absPath(path));

    /* The path is a directory.  Put the Nix expressions in the
       directory in an attribute set, with the file name of each
       expression as the attribute name.  Recurse into subdirectories
       (but keep the attribute set flat, not nested, to make it easier
       for a user to have a ~/.nix-defexpr directory that includes
       some system-wide directory). */
    ATermMap attrs;
    attrs.set(toATerm("_combineChannels"), makeAttrRHS(eTrue, makeNoPos()));
    getAllExprs(state, path, attrs);
    return makeAttrs(attrs);
}


static void loadDerivations(EvalState & state, Path nixExprPath,
    string systemFilter, const ATermMap & autoArgs, DrvInfos & elems)
{
    getDerivations(state, loadSourceExpr(state, nixExprPath), "", autoArgs, elems);

    /* Filter out all derivations not applicable to the current
       system. */
    for (DrvInfos::iterator i = elems.begin(), j; i != elems.end(); i = j) {
        j = i; j++;
        if (systemFilter != "*" && i->system != systemFilter)
            elems.erase(i);
    }
}


static Path getHomeDir()
{
    Path homeDir(getEnv("HOME", ""));
    if (homeDir == "") throw Error("HOME environment variable not set");
    return homeDir;
}


static Path getDefNixExprPath()
{
    return getHomeDir() + "/.nix-defexpr";
}


struct AddPos : TermFun
{
    ATerm operator () (ATerm e)
    {
        ATerm x, y;
        if (matchObsoleteBind(e, x, y))
            return makeBind(x, y, makeNoPos());
        if (matchObsoleteStr(e, x))
            return makeStr(x, ATempty);
        return e;
    }
};


static DrvInfos queryInstalled(EvalState & state, const Path & userEnv)
{
    Path path = userEnv + "/manifest";

    if (!pathExists(path))
        return DrvInfos(); /* not an error, assume nothing installed */

    Expr e = ATreadFromNamedFile(path.c_str());
    if (!e) throw Error(format("cannot read Nix expression from `%1%'") % path);

    /* Compatibility: Bind(x, y) -> Bind(x, y, NoPos). */
    AddPos addPos;
    e = bottomupRewrite(addPos, e);

    DrvInfos elems;
    getDerivations(state, e, "", ATermMap(1), elems);
    return elems;
}


static void createUserEnv(EvalState & state, const DrvInfos & elems,
    const Path & profile, bool keepDerivations)
{
    /* Build the components in the user environment, if they don't
       exist already. */
    PathSet drvsToBuild;
    for (DrvInfos::const_iterator i = elems.begin(); 
         i != elems.end(); ++i)
        /* Call to `isDerivation' is for compatibility with Nix <= 0.7
           user environments. */
        if (i->queryDrvPath(state) != "" &&
            isDerivation(i->queryDrvPath(state)))
            drvsToBuild.insert(i->queryDrvPath(state));

    debug(format("building user environment dependencies"));
    store->buildDerivations(drvsToBuild);

    /* Get the environment builder expression. */
    Expr envBuilder = parseExprFromFile(state,
        nixDataDir + "/nix/corepkgs/buildenv"); /* !!! */

    /* Construct the whole top level derivation. */
    PathSet references;
    ATermList manifest = ATempty;
    ATermList inputs = ATempty;
    for (DrvInfos::const_iterator i = elems.begin(); 
         i != elems.end(); ++i)
    {
        /* Create a pseudo-derivation containing the name, system,
           output path, and optionally the derivation path, as well as
           the meta attributes. */
        Path drvPath = keepDerivations ? i->queryDrvPath(state) : "";
        
        ATermList as = ATmakeList4(
            makeBind(toATerm("type"),
                makeStr("derivation"), makeNoPos()),
            makeBind(toATerm("name"),
                makeStr(i->name), makeNoPos()),
            makeBind(toATerm("system"),
                makeStr(i->system), makeNoPos()),
            makeBind(toATerm("outPath"),
                makeStr(i->queryOutPath(state)), makeNoPos()));
        
        if (drvPath != "") as = ATinsert(as, 
            makeBind(toATerm("drvPath"),
                makeStr(drvPath), makeNoPos()));
        
        if (i->attrs->get(toATerm("meta"))) as = ATinsert(as, 
            makeBind(toATerm("meta"),
                strictEvalExpr(state, i->attrs->get(toATerm("meta"))),
                makeNoPos()));

        manifest = ATinsert(manifest, makeAttrs(as));
        
        inputs = ATinsert(inputs, makeStr(i->queryOutPath(state)));

        /* This is only necessary when installing store paths, e.g.,
           `nix-env -i /nix/store/abcd...-foo'. */
        store->addTempRoot(i->queryOutPath(state));
        store->ensurePath(i->queryOutPath(state));
        
        references.insert(i->queryOutPath(state));
        if (drvPath != "") references.insert(drvPath);
    }

    /* Also write a copy of the list of inputs to the store; we need
       it for future modifications of the environment. */
    Path manifestFile = store->addTextToStore("env-manifest",
        atPrint(canonicaliseExpr(makeList(ATreverse(manifest)))), references);

    Expr topLevel = makeCall(envBuilder, makeAttrs(ATmakeList3(
        makeBind(toATerm("system"),
            makeStr(thisSystem), makeNoPos()),
        makeBind(toATerm("derivations"),
            makeList(ATreverse(manifest)), makeNoPos()),
        makeBind(toATerm("manifest"),
            makeStr(manifestFile, singleton<PathSet>(manifestFile)), makeNoPos())
        )));

    /* Instantiate it. */
    debug(format("evaluating builder expression `%1%'") % topLevel);
    DrvInfo topLevelDrv;
    if (!getDerivation(state, topLevel, topLevelDrv))
        abort();
    
    /* Realise the resulting store expression. */
    debug(format("building user environment"));
    store->buildDerivations(singleton<PathSet>(topLevelDrv.queryDrvPath(state)));

    /* Switch the current user environment to the output path. */
    debug(format("switching to new user environment"));
    Path generation = createGeneration(profile, topLevelDrv.queryOutPath(state));
    switchLink(profile, generation);
}


static int comparePriorities(EvalState & state,
    const DrvInfo & drv1, const DrvInfo & drv2)
{
    int prio1, prio2;
    if (!string2Int(drv1.queryMetaInfo(state, "priority"), prio1)) prio1 = 0;
    if (!string2Int(drv2.queryMetaInfo(state, "priority"), prio2)) prio2 = 0;
    return prio2 - prio1; /* higher number = lower priority, so negate */
}


static DrvInfos filterBySelector(EvalState & state,
    const DrvInfos & allElems,
    const Strings & args, bool newestOnly)
{
    DrvNames selectors = drvNamesFromArgs(args);

    DrvInfos elems;
    set<unsigned int> done;

    for (DrvNames::iterator i = selectors.begin();
         i != selectors.end(); ++i)
    {
        typedef list<std::pair<DrvInfo, unsigned int> > Matches;
        Matches matches;
        unsigned int n = 0;
        for (DrvInfos::const_iterator j = allElems.begin();
             j != allElems.end(); ++j, ++n)
        {
            DrvName drvName(j->name);
            if (i->matches(drvName)) {
                i->hits++;
                matches.push_back(std::pair<DrvInfo, unsigned int>(*j, n));
            }
        }

        /* If `newestOnly', if a selector matches multiple derivations
           with the same name, pick the one with the highest priority.
           If there are multiple derivations with the same priority,
           pick the one with the highest version.  If there are
           multiple derivations with the same priority and name and
           version, then pick the first one. */
        if (newestOnly) {

            /* Map from package names to derivations. */
            typedef map<string, std::pair<DrvInfo, unsigned int> > Newest;
            Newest newest;
            StringSet multiple;

            for (Matches::iterator j = matches.begin(); j != matches.end(); ++j) {
                DrvName drvName(j->first.name);
                int d = 1;

                Newest::iterator k = newest.find(drvName.name);
                
                if (k != newest.end()) {
                    d = comparePriorities(state, j->first, k->second.first);
                    if (d == 0)
                        d = compareVersions(drvName.version, DrvName(k->second.first.name).version);
                }

                if (d > 0) {
                    newest[drvName.name] = *j;
                    multiple.erase(j->first.name);
                } else if (d == 0) {
                    multiple.insert(j->first.name);
                }
            }

            matches.clear();
            for (Newest::iterator j = newest.begin(); j != newest.end(); ++j) {
                if (multiple.find(j->second.first.name) != multiple.end())
                    printMsg(lvlInfo,
                        format("warning: there are multiple derivations named `%1%'; using the first one")
                        % j->second.first.name);
                matches.push_back(j->second);
            }
        }

        /* Insert only those elements in the final list that we
           haven't inserted before. */
        for (Matches::iterator j = matches.begin(); j != matches.end(); ++j)
            if (done.find(j->second) == done.end()) {
                done.insert(j->second);
                elems.push_back(j->first);
            }
    }
            
    /* Check that all selectors have been used. */
    for (DrvNames::iterator i = selectors.begin();
         i != selectors.end(); ++i)
        if (i->hits == 0)
            throw Error(format("selector `%1%' matches no derivations")
                % i->fullName);

    return elems;
}


static void queryInstSources(EvalState & state,
    const InstallSourceInfo & instSource, const Strings & args,
    DrvInfos & elems, bool newestOnly)
{
    InstallSourceType type = instSource.type;
    if (type == srcUnknown && args.size() > 0 && args.front()[0] == '/')
        type = srcStorePaths;
    
    switch (type) {

        /* Get the available user environment elements from the
           derivations specified in a Nix expression, including only
           those with names matching any of the names in `args'. */
        case srcUnknown:
        case srcNixExprDrvs: {

            /* Load the derivations from the (default or specified)
               Nix expression. */
            DrvInfos allElems;
            loadDerivations(state, instSource.nixExprPath,
                instSource.systemFilter, instSource.autoArgs, allElems);

            elems = filterBySelector(state, allElems, args, newestOnly);
    
            break;
        }

        /* Get the available user environment elements from the Nix
           expressions specified on the command line; these should be
           functions that take the default Nix expression file as
           argument, e.g., if the file is `./foo.nix', then the
           argument `x: x.bar' is equivalent to `(x: x.bar)
           (import ./foo.nix)' = `(import ./foo.nix).bar'. */
        case srcNixExprs: {
                
            Expr e1 = loadSourceExpr(state, instSource.nixExprPath);

            for (Strings::const_iterator i = args.begin();
                 i != args.end(); ++i)
            {
                Expr e2 = parseExprFromString(state, *i, absPath("."));
                Expr call = makeCall(e2, e1);
                getDerivations(state, call, "", instSource.autoArgs, elems);
            }
            
            break;
        }
            
        /* The available user environment elements are specified as a
           list of store paths (which may or may not be
           derivations). */
        case srcStorePaths: {

            for (Strings::const_iterator i = args.begin();
                 i != args.end(); ++i)
            {
                assertStorePath(*i);

                DrvInfo elem;
                elem.attrs = boost::shared_ptr<ATermMap>(new ATermMap(0)); /* ugh... */
                string name = baseNameOf(*i);
                string::size_type dash = name.find('-');
                if (dash != string::npos)
                    name = string(name, dash + 1);

                if (isDerivation(*i)) {
                    elem.setDrvPath(*i);
                    elem.setOutPath(findOutput(derivationFromPath(*i), "out"));
                    if (name.size() >= drvExtension.size() &&
                        string(name, name.size() - drvExtension.size()) == drvExtension)
                        name = string(name, 0, name.size() - drvExtension.size());
                }
                else elem.setOutPath(*i);

                elem.name = name;

                elems.push_back(elem);
            }
            
            break;
        }
            
        /* Get the available user environment elements from another
           user environment.  These are then filtered as in the
           `srcNixExprDrvs' case. */
        case srcProfile: {
            elems = filterBySelector(state,
                queryInstalled(state, instSource.profile),
                args, newestOnly);
            break;
        }

        case srcAttrPath: {
            for (Strings::const_iterator i = args.begin();
                 i != args.end(); ++i)
                getDerivations(state,
                    findAlongAttrPath(state, *i, instSource.autoArgs,
                        loadSourceExpr(state, instSource.nixExprPath)),
                    "", instSource.autoArgs, elems);
            break;
        }
    }
}


static void printMissing(EvalState & state, const DrvInfos & elems)
{
    PathSet targets, willBuild, willSubstitute;
    for (DrvInfos::const_iterator i = elems.begin(); i != elems.end(); ++i) {
        Path drvPath = i->queryDrvPath(state);
        if (drvPath != "")
            targets.insert(drvPath);
        else
            targets.insert(i->queryOutPath(state));
    }

    queryMissing(targets, willBuild, willSubstitute);

    if (!willBuild.empty()) {
        printMsg(lvlInfo, format("the following derivations will be built:"));
        for (PathSet::iterator i = willBuild.begin(); i != willBuild.end(); ++i)
            printMsg(lvlInfo, format("  %1%") % *i);
    }

    if (!willSubstitute.empty()) {
        printMsg(lvlInfo, format("the following paths will be substituted:"));
        for (PathSet::iterator i = willSubstitute.begin(); i != willSubstitute.end(); ++i)
            printMsg(lvlInfo, format("  %1%") % *i);
    }
}


static void lockProfile(PathLocks & lock, const Path & profile)
{
    lock.lockPaths(singleton<PathSet>(profile),
        (format("waiting for lock on profile `%1%'") % profile).str());
    lock.setDeletion(true);
}


static void installDerivations(Globals & globals,
    const Strings & args, const Path & profile)
{
    debug(format("installing derivations"));

    /* Get the set of user environment elements to be installed. */
    DrvInfos newElems;
    queryInstSources(globals.state, globals.instSource, args, newElems, true);

    StringSet newNames;
    for (DrvInfos::iterator i = newElems.begin(); i != newElems.end(); ++i) {
        /* `forceName' is a hack to get package names right in some
           one-click installs, namely those where the name used in the
           path is not the one we want (e.g., `java-front' versus
           `java-front-0.9pre15899'). */
        if (globals.forceName != "")
            i->name = globals.forceName;
        newNames.insert(DrvName(i->name).name);
    }

    /* Add in the already installed derivations, unless they have the
       same name as a to-be-installed element. */
    PathLocks lock;
    lockProfile(lock, profile);
    DrvInfos installedElems = queryInstalled(globals.state, profile);

    DrvInfos allElems(newElems);
    for (DrvInfos::iterator i = installedElems.begin();
         i != installedElems.end(); ++i)
    {
        DrvName drvName(i->name);
        MetaInfo meta = i->queryMetaInfo(globals.state);
        if (!globals.preserveInstalled &&
            newNames.find(drvName.name) != newNames.end() &&
            meta["keep"] != "true")
            printMsg(lvlInfo,
                format("replacing old `%1%'") % i->name);
        else
            allElems.push_back(*i);
    }

    for (DrvInfos::iterator i = newElems.begin(); i != newElems.end(); ++i)
        printMsg(lvlInfo,
            format("installing `%1%'") % i->name);
        
    if (globals.dryRun) {
        printMissing(globals.state, newElems);
        return;
    }

    createUserEnv(globals.state, allElems,
        profile, globals.keepDerivations);
}


static void opInstall(Globals & globals,
    Strings opFlags, Strings opArgs)
{
    if (opFlags.size() > 0)
        throw UsageError(format("unknown flag `%1%'") % opFlags.front());

    installDerivations(globals, opArgs, globals.profile);
}


typedef enum { utLt, utLeq, utEq, utAlways } UpgradeType;


static void upgradeDerivations(Globals & globals,
    const Strings & args, UpgradeType upgradeType)
{
    debug(format("upgrading derivations"));

    /* Upgrade works as follows: we take all currently installed
       derivations, and for any derivation matching any selector, look
       for a derivation in the input Nix expression that has the same
       name and a higher version number. */

    /* Load the currently installed derivations. */
    PathLocks lock;
    lockProfile(lock, globals.profile);
    DrvInfos installedElems = queryInstalled(globals.state, globals.profile);

    /* Fetch all derivations from the input file. */
    DrvInfos availElems;
    queryInstSources(globals.state, globals.instSource, args, availElems, false);

    /* Go through all installed derivations. */
    DrvInfos newElems;
    for (DrvInfos::iterator i = installedElems.begin();
         i != installedElems.end(); ++i)
    {
        DrvName drvName(i->name);

        MetaInfo meta = i->queryMetaInfo(globals.state);
        if (meta["keep"] == "true") {
            newElems.push_back(*i);
            continue;
        }

        /* Find the derivation in the input Nix expression with the
           same name that satisfies the version constraints specified
           by upgradeType.  If there are multiple matches, take the
           one with the highest priority.  If there are still multiple
           matches, take the one with the highest version. */
        DrvInfos::iterator bestElem = availElems.end();
        DrvName bestName;
        for (DrvInfos::iterator j = availElems.begin();
             j != availElems.end(); ++j)
        {
            DrvName newName(j->name);
            if (newName.name == drvName.name) {
                int d = comparePriorities(globals.state, *i, *j);
                if (d == 0) d = compareVersions(drvName.version, newName.version);
                if (upgradeType == utLt && d < 0 ||
                    upgradeType == utLeq && d <= 0 ||
                    upgradeType == utEq && d == 0 ||
                    upgradeType == utAlways)
                {
                    int d2 = -1;
                    if (bestElem != availElems.end()) {
                        d2 = comparePriorities(globals.state, *bestElem, *j);
                        if (d2 == 0) d2 = compareVersions(bestName.version, newName.version);
                    }
                    if (d2 < 0) {
                        bestElem = j;
                        bestName = newName;
                    }
                }
            }
        }

        if (bestElem != availElems.end() &&
            i->queryOutPath(globals.state) !=
                bestElem->queryOutPath(globals.state))
        {
            printMsg(lvlInfo,
                format("upgrading `%1%' to `%2%'")
                % i->name % bestElem->name);
            newElems.push_back(*bestElem);
        } else newElems.push_back(*i);
    }
    
    if (globals.dryRun) {
        printMissing(globals.state, newElems);
        return;
    }

    createUserEnv(globals.state, newElems,
        globals.profile, globals.keepDerivations);
}


static void opUpgrade(Globals & globals,
    Strings opFlags, Strings opArgs)
{
    UpgradeType upgradeType = utLt;
    for (Strings::iterator i = opFlags.begin();
         i != opFlags.end(); ++i)
        if (*i == "--lt") upgradeType = utLt;
        else if (*i == "--leq") upgradeType = utLeq;
        else if (*i == "--eq") upgradeType = utEq;
        else if (*i == "--always") upgradeType = utAlways;
        else throw UsageError(format("unknown flag `%1%'") % *i);

    upgradeDerivations(globals, opArgs, upgradeType);
}


static void setMetaFlag(EvalState & state, DrvInfo & drv,
    const string & name, const string & value)
{
    MetaInfo meta = drv.queryMetaInfo(state);
    meta[name] = value;
    drv.setMetaInfo(meta);
}


static void opSetFlag(Globals & globals,
    Strings opFlags, Strings opArgs)
{
    if (opFlags.size() > 0)
        throw UsageError(format("unknown flag `%1%'") % opFlags.front());
    if (opArgs.size() < 2)
        throw UsageError("not enough arguments to `--set-flag'");

    Strings::iterator arg = opArgs.begin();
    string flagName = *arg++;
    string flagValue = *arg++;
    DrvNames selectors = drvNamesFromArgs(Strings(arg, opArgs.end()));

    /* Load the currently installed derivations. */
    PathLocks lock;
    lockProfile(lock, globals.profile);
    DrvInfos installedElems = queryInstalled(globals.state, globals.profile);

    /* Update all matching derivations. */
    for (DrvInfos::iterator i = installedElems.begin();
         i != installedElems.end(); ++i)
    {
        DrvName drvName(i->name);
        for (DrvNames::iterator j = selectors.begin();
             j != selectors.end(); ++j)
            if (j->matches(drvName)) {
                printMsg(lvlInfo,
                    format("setting flag on `%1%'") % i->name);
                setMetaFlag(globals.state, *i, flagName, flagValue);
                break;
            }
    }

    /* Write the new user environment. */
    createUserEnv(globals.state, installedElems,
        globals.profile, globals.keepDerivations);
}


static void opSet(Globals & globals,
    Strings opFlags, Strings opArgs)
{
    if (opFlags.size() > 0)
        throw UsageError(format("unknown flag `%1%'") % opFlags.front());

    DrvInfos elems;
    queryInstSources(globals.state, globals.instSource, opArgs, elems, true);

    if (elems.size() != 1)
        throw Error("--set requires exactly one derivation");
    
    DrvInfo & drv(elems.front());

    if (drv.queryDrvPath(globals.state) != "")
        store->buildDerivations(singleton<PathSet>(drv.queryDrvPath(globals.state)));
    else
        store->ensurePath(drv.queryOutPath(globals.state));

    debug(format("switching to new user environment"));
    Path generation = createGeneration(globals.profile, drv.queryOutPath(globals.state));
    switchLink(globals.profile, generation);
}


static void uninstallDerivations(Globals & globals, DrvNames & selectors,
    Path & profile)
{
    PathLocks lock;
    lockProfile(lock, profile);
    DrvInfos installedElems = queryInstalled(globals.state, profile);
    DrvInfos newElems;

    for (DrvInfos::iterator i = installedElems.begin();
         i != installedElems.end(); ++i)
    {
        DrvName drvName(i->name);
        bool found = false;
        for (DrvNames::iterator j = selectors.begin();
             j != selectors.end(); ++j)
            if (j->matches(drvName)) {
                printMsg(lvlInfo,
                    format("uninstalling `%1%'") % i->name);
                found = true;
                break;
            }
        if (!found) newElems.push_back(*i);
    }

    if (globals.dryRun) return;

    createUserEnv(globals.state, newElems,
        profile, globals.keepDerivations);
}


static void opUninstall(Globals & globals,
    Strings opFlags, Strings opArgs)
{
    if (opFlags.size() > 0)
        throw UsageError(format("unknown flag `%1%'") % opFlags.front());

    DrvNames drvNames = drvNamesFromArgs(opArgs);

    uninstallDerivations(globals, drvNames,
        globals.profile);
}


static bool cmpChars(char a, char b)
{
    return toupper(a) < toupper(b);
}


static bool cmpElemByName(const DrvInfo & a, const DrvInfo & b)
{
    return lexicographical_compare(
        a.name.begin(), a.name.end(),
        b.name.begin(), b.name.end(), cmpChars);
}


typedef list<Strings> Table;


void printTable(Table & table)
{
    unsigned int nrColumns = table.size() > 0 ? table.front().size() : 0;
    
    vector<unsigned int> widths;
    widths.resize(nrColumns);
    
    for (Table::iterator i = table.begin(); i != table.end(); ++i) {
        assert(i->size() == nrColumns);
        Strings::iterator j;
        unsigned int column;
        for (j = i->begin(), column = 0; j != i->end(); ++j, ++column)
            if (j->size() > widths[column]) widths[column] = j->size();
    }

    for (Table::iterator i = table.begin(); i != table.end(); ++i) { 
        Strings::iterator j;
        unsigned int column;
        for (j = i->begin(), column = 0; j != i->end(); ++j, ++column)
        {
            cout << *j;
            if (column < nrColumns - 1)
                cout << string(widths[column] - j->size() + 2, ' ');
        }
        cout << std::endl;
    }
}


/* This function compares the version of a element against the
   versions in the given set of elements.  `cvLess' means that only
   lower versions are in the set, `cvEqual' means that at most an
   equal version is in the set, and `cvGreater' means that there is at
   least one element with a higher version in the set.  `cvUnavail'
   means that there are no elements with the same name in the set. */

typedef enum { cvLess, cvEqual, cvGreater, cvUnavail } VersionDiff;

static VersionDiff compareVersionAgainstSet(
    const DrvInfo & elem, const DrvInfos & elems, string & version)
{
    DrvName name(elem.name);
    
    VersionDiff diff = cvUnavail;
    version = "?";
    
    for (DrvInfos::const_iterator i = elems.begin(); i != elems.end(); ++i) {
        DrvName name2(i->name);
        if (name.name == name2.name) {
            int d = compareVersions(name.version, name2.version);
            if (d < 0) {
                diff = cvGreater;
                version = name2.version;
            }
            else if (diff != cvGreater && d == 0) {
                diff = cvEqual;
                version = name2.version;
            }
            else if (diff != cvGreater && diff != cvEqual && d > 0) {
                diff = cvLess;
                if (version == "" || compareVersions(version, name2.version) < 0)
                    version = name2.version;
            }
        }
    }

    return diff;
}


static string colorString(const string & s)
{
    if (!isatty(STDOUT_FILENO)) return s;
    return "\e[1;31m" + s + "\e[0m";
}


static void opQuery(Globals & globals,
    Strings opFlags, Strings opArgs)
{
    typedef vector< map<string, string> > ResultSet;
        
    bool printStatus = false;
    bool printName = true;
    bool printAttrPath = false;
    bool printSystem = false;
    bool printDrvPath = false;
    bool printOutPath = false;
    bool printDescription = false;
    bool printMeta = false;
    bool prebuiltOnly = false;
    bool compareVersions = false;
    bool xmlOutput = false;

    enum { sInstalled, sAvailable } source = sInstalled;

    readOnlyMode = true; /* makes evaluation a bit faster */

    for (Strings::iterator i = opFlags.begin();
         i != opFlags.end(); ++i)
        if (*i == "--status" || *i == "-s") printStatus = true;
        else if (*i == "--no-name") printName = false;
        else if (*i == "--system") printSystem = true;
        else if (*i == "--description") printDescription = true;
        else if (*i == "--compare-versions" || *i == "-c") compareVersions = true;
        else if (*i == "--drv-path") printDrvPath = true;
        else if (*i == "--out-path") printOutPath = true;
        else if (*i == "--meta") printMeta = true;
        else if (*i == "--installed") source = sInstalled;
        else if (*i == "--available" || *i == "-a") source = sAvailable;
        else if (*i == "--prebuilt-only" || *i == "-b") prebuiltOnly = true;
        else if (*i == "--xml") xmlOutput = true;
        else throw UsageError(format("unknown flag `%1%'") % *i);

    if (globals.instSource.type == srcAttrPath) printAttrPath = true; /* hack */

    if (opArgs.size() == 0) {
        printMsg(lvlInfo, "warning: you probably meant to specify the argument '*' to show all packages");
    }

    
    /* Obtain derivation information from the specified source. */
    DrvInfos availElems, installedElems;

    if (source == sInstalled || compareVersions || printStatus) {
        installedElems = queryInstalled(globals.state, globals.profile);
    }

    if (source == sAvailable || compareVersions) {
        loadDerivations(globals.state, globals.instSource.nixExprPath,
            globals.instSource.systemFilter, globals.instSource.autoArgs,
            availElems);
    }

    DrvInfos elems = filterBySelector(globals.state,
        source == sInstalled ? installedElems : availElems,
        opArgs, false);
    
    DrvInfos & otherElems(source == sInstalled ? availElems : installedElems);

    
    /* Sort them by name. */
    /* !!! */
    vector<DrvInfo> elems2;
    for (DrvInfos::iterator i = elems.begin(); i != elems.end(); ++i)
        elems2.push_back(*i);
    sort(elems2.begin(), elems2.end(), cmpElemByName);

    
    /* We only need to know the installed paths when we are querying
       the status of the derivation. */
    PathSet installed; /* installed paths */
    
    if (printStatus) {
        for (DrvInfos::iterator i = installedElems.begin();
             i != installedElems.end(); ++i)
            installed.insert(i->queryOutPath(globals.state));
    }

    
    /* Print the desired columns, or XML output. */
    Table table;
    std::ostringstream dummy;
    XMLWriter xml(true, *(xmlOutput ? &cout : &dummy));
    XMLOpenElement xmlRoot(xml, "items");
    
    for (vector<DrvInfo>::iterator i = elems2.begin();
         i != elems2.end(); ++i)
    {
        try {

            /* For table output. */
            Strings columns;

            /* For XML output. */
            XMLAttrs attrs;

            if (prebuiltOnly) {
                if (!store->isValidPath(i->queryOutPath(globals.state)) &&
                    !store->hasSubstitutes(i->queryOutPath(globals.state)))
                    continue;
            }
        
            if (printStatus) {
                bool hasSubs = store->hasSubstitutes(i->queryOutPath(globals.state));
                bool isInstalled = installed.find(i->queryOutPath(globals.state)) != installed.end();
                bool isValid = store->isValidPath(i->queryOutPath(globals.state));
                if (xmlOutput) {
                    attrs["installed"] = isInstalled ? "1" : "0";
                    attrs["valid"] = isValid ? "1" : "0";
                    attrs["substitutable"] = hasSubs ? "1" : "0";
                } else
                    columns.push_back(
                        (string) (isInstalled ? "I" : "-")
                        + (isValid ? "P" : "-")
                        + (hasSubs ? "S" : "-"));
            }

            if (xmlOutput)
                attrs["attrPath"] = i->attrPath;
            else if (printAttrPath)
                columns.push_back(i->attrPath);

            if (xmlOutput)
                attrs["name"] = i->name;
            else if (printName)
                columns.push_back(i->name);

            if (compareVersions) {
                /* Compare this element against the versions of the
                   same named packages in either the set of available
                   elements, or the set of installed elements.  !!!
                   This is O(N * M), should be O(N * lg M). */
                string version;
                VersionDiff diff = compareVersionAgainstSet(*i, otherElems, version);

                char ch;
                switch (diff) {
                    case cvLess: ch = '>'; break;
                    case cvEqual: ch = '='; break;
                    case cvGreater: ch = '<'; break;
                    case cvUnavail: ch = '-'; break;
                    default: abort();
                }

                if (xmlOutput) {
                    if (diff != cvUnavail) {
                        attrs["versionDiff"] = ch;
                        attrs["maxComparedVersion"] = version;
                    }
                } else {
                    string column = (string) "" + ch + " " + version;
                    if (diff == cvGreater) column = colorString(column);
                    columns.push_back(column);
                }
            }

            if (xmlOutput) {
                if (i->system != "") attrs["system"] = i->system;
            }
            else if (printSystem) 
                columns.push_back(i->system);

            if (printDrvPath) {
                string drvPath = i->queryDrvPath(globals.state);
                if (xmlOutput) {
                    if (drvPath != "") attrs["drvPath"] = drvPath;
                } else
                    columns.push_back(drvPath == "" ? "-" : drvPath);
            }
        
            if (printOutPath) {
                string outPath = i->queryOutPath(globals.state);
                if (xmlOutput) {
                    if (outPath != "") attrs["outPath"] = outPath;
                } else
                    columns.push_back(outPath);
            }

            if (printDescription) {
                MetaInfo meta = i->queryMetaInfo(globals.state);
                string descr = meta["description"];
                if (xmlOutput) {
                    if (descr != "") attrs["description"] = descr;
                } else
                    columns.push_back(descr);
            }

            if (xmlOutput)
                if (printMeta) {
                    XMLOpenElement item(xml, "item", attrs);
                    MetaInfo meta = i->queryMetaInfo(globals.state);
                    for (MetaInfo::iterator j = meta.begin(); j != meta.end(); ++j) {
                        XMLAttrs attrs2;
                        attrs2["name"] = j->first;
                        attrs2["value"] = j->second;
                        xml.writeEmptyElement("meta", attrs2);
                    }
                }
                else
                    xml.writeEmptyElement("item", attrs);
            else
                table.push_back(columns);

            cout.flush();

        } catch (AssertionError & e) {
            printMsg(lvlTalkative, format("skipping derivation named `%1%' which gives an assertion failure") % i->name);
        }
    }

    if (!xmlOutput) printTable(table);
}


static void opSwitchProfile(Globals & globals,
    Strings opFlags, Strings opArgs)
{
    if (opFlags.size() > 0)
        throw UsageError(format("unknown flag `%1%'") % opFlags.front());
    if (opArgs.size() != 1)
        throw UsageError(format("exactly one argument expected"));

    Path profile = absPath(opArgs.front());
    Path profileLink = getHomeDir() + "/.nix-profile";

    switchLink(profileLink, profile);
}


static const int prevGen = -2;


static void switchGeneration(Globals & globals, int dstGen)
{
    PathLocks lock;
    lockProfile(lock, globals.profile);
    
    int curGen;
    Generations gens = findGenerations(globals.profile, curGen);

    Generation dst;
    for (Generations::iterator i = gens.begin(); i != gens.end(); ++i)
        if ((dstGen == prevGen && i->number < curGen) ||
            (dstGen >= 0 && i->number == dstGen))
            dst = *i;

    if (!dst)
        if (dstGen == prevGen)
            throw Error(format("no generation older than the current (%1%) exists")
                % curGen);
        else
            throw Error(format("generation %1% does not exist") % dstGen);

    printMsg(lvlInfo, format("switching from generation %1% to %2%")
        % curGen % dst.number);
    
    if (globals.dryRun) return;
    
    switchLink(globals.profile, dst.path);
}


static void opSwitchGeneration(Globals & globals,
    Strings opFlags, Strings opArgs)
{
    if (opFlags.size() > 0)
        throw UsageError(format("unknown flag `%1%'") % opFlags.front());
    if (opArgs.size() != 1)
        throw UsageError(format("exactly one argument expected"));

    int dstGen;
    if (!string2Int(opArgs.front(), dstGen))
        throw UsageError(format("expected a generation number"));

    switchGeneration(globals, dstGen);
}


static void opRollback(Globals & globals,
    Strings opFlags, Strings opArgs)
{
    if (opFlags.size() > 0)
        throw UsageError(format("unknown flag `%1%'") % opFlags.front());
    if (opArgs.size() != 0)
        throw UsageError(format("no arguments expected"));

    switchGeneration(globals, prevGen);
}


static void opListGenerations(Globals & globals,
    Strings opFlags, Strings opArgs)
{
    if (opFlags.size() > 0)
        throw UsageError(format("unknown flag `%1%'") % opFlags.front());
    if (opArgs.size() != 0)
        throw UsageError(format("no arguments expected"));

    PathLocks lock;
    lockProfile(lock, globals.profile);
    
    int curGen;
    Generations gens = findGenerations(globals.profile, curGen);

    for (Generations::iterator i = gens.begin(); i != gens.end(); ++i) {
        tm t;
        if (!localtime_r(&i->creationTime, &t)) throw Error("cannot convert time");
        cout << format("%|4|   %|4|-%|02|-%|02| %|02|:%|02|:%|02|   %||\n")
            % i->number
            % (t.tm_year + 1900) % (t.tm_mon + 1) % t.tm_mday
            % t.tm_hour % t.tm_min % t.tm_sec
            % (i->number == curGen ? "(current)" : "");
    }
}


static void deleteGeneration2(const Path & profile, unsigned int gen)
{
    printMsg(lvlInfo, format("removing generation %1%") % gen);
    deleteGeneration(profile, gen);
}


static void opDeleteGenerations(Globals & globals,
    Strings opFlags, Strings opArgs)
{
    if (opFlags.size() > 0)
        throw UsageError(format("unknown flag `%1%'") % opFlags.front());

    PathLocks lock;
    lockProfile(lock, globals.profile);
    
    int curGen;
    Generations gens = findGenerations(globals.profile, curGen);

    for (Strings::iterator i = opArgs.begin(); i != opArgs.end(); ++i) {

        if (*i == "old") {
            for (Generations::iterator j = gens.begin(); j != gens.end(); ++j)
                if (j->number != curGen)
                    deleteGeneration2(globals.profile, j->number);
        }

        else {
            int n;
            if (!string2Int(*i, n) || n < 0)
                throw UsageError(format("invalid generation specifier `%1%'")  % *i);
            bool found = false;
            for (Generations::iterator j = gens.begin(); j != gens.end(); ++j) {
                if (j->number == n) {
                    deleteGeneration2(globals.profile, j->number);
                    found = true;
                    break;
                }
            }
            if (!found)
                printMsg(lvlError, format("generation %1% does not exist") % n);
        }
    }
}


static string needArg(Strings::iterator & i,
    Strings & args, const string & arg)
{
    if (i == args.end()) throw UsageError(
        format("`%1%' requires an argument") % arg);
    return *i++;
}


void run(Strings args)
{
    Strings opFlags, opArgs;
    Operation op = 0;
    
    Globals globals;
    
    globals.instSource.type = srcUnknown;
    globals.instSource.nixExprPath = getDefNixExprPath();
    globals.instSource.systemFilter = thisSystem;
    
    globals.dryRun = false;
    globals.preserveInstalled = false;

    globals.keepDerivations =
        queryBoolSetting("env-keep-derivations", false);
    
    for (Strings::iterator i = args.begin(); i != args.end(); ) {
        string arg = *i++;

        Operation oldOp = op;

        if (arg == "--install" || arg == "-i")
            op = opInstall;
        else if (arg == "--from-expression" || arg == "-E")
            globals.instSource.type = srcNixExprs;
        else if (arg == "--from-profile") {
            globals.instSource.type = srcProfile;
            globals.instSource.profile = needArg(i, args, arg);
        }
        else if (arg == "--attr" || arg == "-A")
            globals.instSource.type = srcAttrPath;
        else if (parseOptionArg(arg, i, args.end(),
                     globals.state, globals.instSource.autoArgs))
            ;
        else if (arg == "--force-name") // undocumented flag for nix-install-package
            globals.forceName = needArg(i, args, arg);
        else if (arg == "--uninstall" || arg == "-e")
            op = opUninstall;
        else if (arg == "--upgrade" || arg == "-u")
            op = opUpgrade;
        else if (arg == "--set-flag")
            op = opSetFlag;
        else if (arg == "--set")
            op = opSet;
        else if (arg == "--query" || arg == "-q")
            op = opQuery;
        else if (arg == "--profile" || arg == "-p")
            globals.profile = absPath(needArg(i, args, arg));
        else if (arg == "--file" || arg == "-f")
            globals.instSource.nixExprPath = absPath(needArg(i, args, arg));
        else if (arg == "--switch-profile" || arg == "-S")
            op = opSwitchProfile;
        else if (arg == "--switch-generation" || arg == "-G")
            op = opSwitchGeneration;
        else if (arg == "--rollback")
            op = opRollback;
        else if (arg == "--list-generations")
            op = opListGenerations;
        else if (arg == "--delete-generations")
            op = opDeleteGenerations;
        else if (arg == "--dry-run") {
            printMsg(lvlInfo, "(dry run; not doing anything)");
            globals.dryRun = true;
        }
        else if (arg == "--preserve-installed" || arg == "-P")
            globals.preserveInstalled = true;
        else if (arg == "--system-filter")
            globals.instSource.systemFilter = needArg(i, args, arg);
        else if (arg[0] == '-')
            opFlags.push_back(arg);
        else
            opArgs.push_back(arg);

        if (oldOp && oldOp != op)
            throw UsageError("only one operation may be specified");
    }

    if (!op) throw UsageError("no operation specified");

    if (globals.profile == "") {
        Path profileLink = getHomeDir() + "/.nix-profile";
        globals.profile = pathExists(profileLink)
            ? absPath(readLink(profileLink), dirOf(profileLink))
            : canonPath(nixStateDir + "/profiles/default");
    }
    
    store = openStore();

    op(globals, opFlags, opArgs);

    printEvalStats(globals.state);
}


string programId = "nix-env";