I have a bash script I use to run a few python and C++ programs in a sequence.

Each program takes in some input parameters which I define in the bash script. So as an example I run the program like this:

echo $param1 $param2 $param3 | python foo.py

The python program outputs some values which we use as input for later programs. The thing, like I said above, is that I do not need to run the python program if I store the values in some file and read them from there.

So my question then is. Is there some generic tool that achieves this feature? That is, is there some program called 'bar' which I could run like

bar $param1 $param2 $param3 "python foo.py"

which would check if a cache file exists, if yes it would check if the program has been run with the given parameters and if yes it would output the cached output values instead of running the program again.

Edit: The name of the log file could also be provided as input of course.

  • 6
    I'd use make. Just write the input parameters to a file and make will handle all the dependencies and only run the tasks whose input has changed.
    – choroba
    May 6, 2016 at 9:53
  • Thanks, I'll consider that, do you know of any examples I could use for reference?
    – Haffi112
    May 6, 2016 at 10:09
  • Why wouldn't you simply re-write the python script(s) to accept input parameters?
    – Thomas N
    May 6, 2016 at 16:38
  • They do, but then I have to write in each python script this functionality. The other solution is cleaner in my opinion since it does not require any extra complexity in the programs I'm running and it could be applied to any executable which fulfils the input/output conditions.
    – Haffi112
    May 7, 2016 at 9:25
  • The answers on stackoverflow.com/questions/11900239/… include a few simple scripts to achieve this.
    – JigglyNaga
    May 16, 2016 at 12:33

4 Answers 4


I just got nerd-sniped into writing a rather complete script for this; latest version is at https://gist.github.com/akorn/51ee2fe7d36fa139723c851d87e56096.

Advantages over sivann's shell implementation:

  • also takes envvars into account when computing cache key;
  • completely avoids race conditions using locking instead of relying on a random wait
  • better performance when called in tight loop due to fewer forks
  • also caches stderr
  • completely transparent: prints nothing; doesn't prevent parallel execution of same command; just runs command uncached if there is a problem with the cache
  • configurable via envvars and command line switches
  • can prune its cache (remove all obsolete entries)

Disadvantage: written in zsh, not bash.

# Purpose: run speficied command with specified arguments and cache result. If cache is fresh enough, don't run command again but return cached output.
# Also cache exit status and stderr.
# Copyright (c) 2019-2020 András Korn; License: GPLv3

# Use silly long variable names to avoid clashing with whatever the invoked program might use

function usage() {
    echo "Usage: runcached [--ttl <max cache age>] [--cache-dir <cache directory>]"
    echo "       [--ignore-env] [--ignore-pwd] [--help] [--prune-cache]"
    echo "       [--] command [arg1 [arg2 ...]]"
    echo "Run 'command' with the specified args and cache stdout, stderr and exit"
    echo "status. If you run the same command again and the cache is fresh, cached"
    echo "data is returned and the command is not actually run."
    echo "Normally, all exported environment variables as well as the current working"
    echo "directory are included in the cache key. The --ignore options disable this."
    echo "The OLDPWD variable is always ignored."
    echo "--prune-cache deletes all cache entries older than the maximum age. There is"
    echo "no other mechanism to prevent the cache growing without bounds."
    echo "The default cache directory is ${RUNCACHED_CACHE_DIR}."
    echo "Maximum cache age defaults to ${RUNCACHED_MAX_AGE}."
    echo "CAVEATS:"
    echo "Side effects of 'command' are obviously not cached."
    echo "There is no cache invalidation logic except cache age (specified in seconds)."
    echo "If the cache can't be created, the command is run uncached."
    echo "This script is always silent; any output comes from the invoked command. You"
    echo "may thus not notice errors creating the cache and such."
    echo "stdout and stderr streams are saved separately. When both are written to a"
    echo "terminal from cache, they will almost certainly be interleaved differently"
    echo "than originally. Ordering of messages within the two streams is preserved."
    exit 0

while [[ -n "$1" ]]; do
    case "$1" in
        --ttl)      RUNCACHED_MAX_AGE="$2"; shift 2;;
        --cache-dir)    RUNCACHED_CACHE_DIR="$2"; shift 2;;
        --ignore-env)   RUNCACHED_IGNORE_ENV=1; shift;;
        --ignore-pwd)   RUNCACHED_IGNORE_PWD=1; shift;;
        --prune-cache)  RUNCACHED_PRUNE=1; shift;;
        --help)     usage;;
        --)     shift; break;;
        *)      break;;

zmodload zsh/datetime
zmodload zsh/stat
zmodload zsh/system
zmodload zsh/files

# the built-in mv doesn't fall back to copy if renaming fails due to EXDEV;
# since the cache directory is likely on a different fs than the tmp
# directory, this is an important limitation, so we use /bin/mv instead
disable mv  

mkdir -p "$RUNCACHED_CACHE_DIR" >/dev/null 2>/dev/null

((RUNCACHED_PRUNE)) && find "$RUNCACHED_CACHE_DIR/." -maxdepth 1 -type f \! -newermt @$[EPOCHSECONDS-RUNCACHED_MAX_AGE] -delete 2>/dev/null

[[ -n "$@" ]] || exit 0 # if no command specified, exit silently

    # Almost(?) nothing uses OLDPWD, but taking it into account potentially reduces cache efficency.
    # Thus, we ignore it for the purpose of coming up with a cache key.
    unset OLDPWD
    echo -E "$@"
) | md5sum | read RUNCACHED_CACHE_KEY RUNCACHED__crap__
# make the cache dir hashed unless a cache file already exists (created by a previous version that didn't use hashed dirs)
if ! [[ -f $RUNCACHED_CACHE_DIR/$RUNCACHED_CACHE_KEY.exitstatus ]]; then
    mkdir -p "$RUNCACHED_CACHE_DIR/${RUNCACHED_CACHE_KEY:h}" >/dev/null 2>/dev/null

# If we can't obtain a lock, we want to run uncached; otherwise
# 'runcached' wouldn't be transparent because it would prevent
# parallel execution of several instances of the same command.
# Locking is necessary to avoid races between the mv(1) command
# below replacing stderr with a newer version and another instance
# of runcached using a newer stdout with the older stderr.
if zsystem flock -t 0 $RUNCACHED_CACHE_DIR/$RUNCACHED_CACHE_KEY.lock 2>/dev/null; then
    if [[ -f $RUNCACHED_CACHE_DIR/$RUNCACHED_CACHE_KEY.stdout ]]; then
        if [[ $[EPOCHSECONDS-$(zstat +mtime $RUNCACHED_CACHE_DIR/$RUNCACHED_CACHE_KEY.stdout)] -le $RUNCACHED_MAX_AGE ]]; then
            cat $RUNCACHED_CACHE_DIR/$RUNCACHED_CACHE_KEY.stderr >&2 &
            exit $(<$RUNCACHED_CACHE_DIR/$RUNCACHED_CACHE_KEY.exitstatus)
            rm -f $RUNCACHED_CACHE_DIR/$RUNCACHED_CACHE_KEY.{stdout,stderr,exitstatus} 2>/dev/null

    # only reached if cache didn't exist or was too old
    if [[ -d $RUNCACHED_CACHE_DIR/. ]]; then
        RUNCACHED_tempdir=$(mktemp -d 2>/dev/null)
        if [[ -d $RUNCACHED_tempdir/. ]]; then
            $@ >&1 >$RUNCACHED_tempdir/${RUNCACHED_CACHE_KEY:t}.stdout 2>&2 2>$RUNCACHED_tempdir/${RUNCACHED_CACHE_KEY:t}.stderr
            echo $RUNCACHED_ret >$RUNCACHED_tempdir/${RUNCACHED_CACHE_KEY:t}.exitstatus 2>/dev/null
            mv $RUNCACHED_tempdir/${RUNCACHED_CACHE_KEY:t}.{stdout,stderr,exitstatus} $RUNCACHED_CACHE_DIR/${RUNCACHED_CACHE_KEY:h} 2>/dev/null
            rmdir $RUNCACHED_tempdir 2>/dev/null
            exit $RUNCACHED_ret

# only reached if cache not created successfully or lock couldn't be obtained
exec $@
  • That's beautiful, thank you! :)
    – Haffi112
    Jun 25, 2019 at 9:11

An implementation exists here: https://bitbucket.org/sivann/runcached/src Caches executable path, output, exit code, remembers arguments. Configurable expiration. Implemented in bash, C, python, choose whatever suits you. The "bash" version is somewhat limited.


For Bash users I created bash-cache, which provides a similar feature set to András Korn's zsh approach. It supports env vars, avoids races, caches stderr and exit codes, invalidates stale data, support asynchronous warming and mutex locks, and more.

For example I think you can implement what you're describing like so:

foo() {
  python foo.py <<<"$*" # this writes all arguments to the python stdin
} && bc::cache foo

Which you'd then invoke as:

foo param1 param2 param3

And foo will cache the results and reuse them in subsequent calls.

See my earlier answer for more details.


Simple python solution based on existing packages (joblib.Memory for memoizing):


import sys, os
import joblib
import subprocess

mem = joblib.Memory('.', verbose=False)
def run_cmd(args, env):
    process = subprocess.run(args, capture_output=True, text=True)
    return process.stdout, process.stderr, process.returncode

stdout, stderr, returncode = run_cmd(sys.argv[1:], dict(os.environ))

Usage example:

python cmdcache.py ls

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