which
is non-standard. Use builtin command -v
instead.
This is an optional suggestion. It must be explicitly enabled with a directive enable=deprecate-which
in a # shellcheck
comment or .shellcheckrc
Problematic code:
which grep
Correct code:
# For the path of a single, unaliased, external command,
# or to check whether this will just "run" in this shell:
command -v grep
# To check whether commands exist, without obtaining a reusable path:
hash grep
Rationale:
which
is a non-standard, external tool that locates an executable in PATH. command -v
is a POSIX standard builtin, which uses the same lookup mechanism that the shell itself would.
Exceptions:
This check is opt-in only in 0.7.1+, and you may choose to ignore it in earlier versions. which
is very common, and some prefer its executable-or-nothing behavior over command -v
's handling of builtins, functions and aliases.
Caveats:
command -v
does not check ALL parameters
command -v
succeeds (with exit code 0) if any command exists:
# grep is in /usr/bin/grep
# foobar is not in path
#
$ command -v -- grep foobar; echo $?
0
In the above example, it should have failed and exited with 1 unless all commands
exist, if it were to be a replacement for which
. Other problems associated with
command
include its inclusion of builtins, aliases, and functions.
An alternative is:
$ hash <file1> <file2>
Which observes the standard behaviour of failures.
To obtain a path, type -p
can be used instead. Like command -v
, it has a similarly
quirky behavior with builtins, aliases, and functions, although this is
arguably milder since it would print nothing for these cases. The failure condition is
similar to hash
.
Related resources:
shellcheck
issue: #1162 command -v is not a direct replacement for which (Discussion)- Check if a program exists from a Bash script on StackOverflow.