How can I check if a program exists from a Bash script?

Question

How would I validate that a program exists, in a way that will either return an error and exit, or continue with the script?

It seems like it should be easy, but it\

Answer 1

To use hash, as @lhunath suggests, in a Bash script:

hash foo &> /dev/null
if [ $? -eq 1 ]; then
    echo >&2 "foo not found."
fi

This script runs hash and then checks if the exit code of the most recent command, the value stored in $?, is equal to 1. If hash doesn't find foo, the exit code will be 1. If foo is present, the exit code will be 0.

&> /dev/null redirects standard error and standard output from hash so that it doesn't appear onscreen and echo >&2 writes the message to standard error.

Answer 2

Check for multiple dependencies and inform status to end users

for cmd in latex pandoc; do
  printf '%-10s' "$cmd"
  if hash "$cmd" 2>/dev/null; then
    echo OK
  else
    echo missing
  fi
done

Sample output:

latex     OK
pandoc    missing

Adjust the 10 to the maximum command length. It is not automatic, because I don't see a non-verbose POSIX way to do it: How can I align the columns of a space separated table in Bash?

Check if some apt packages are installed with dpkg -s and install them otherwise.

See: Check if an apt-get package is installed and then install it if it's not on Linux

It was previously mentioned at: How can I check if a program exists from a Bash script?

Answer 3

I had to check if Git was installed as part of deploying our CI server. My final Bash script was as follows (Ubuntu server):

if ! builtin type -p git &>/dev/null; then
  sudo apt-get -y install git-core
fi

Answer 4

I agree with lhunath to discourage use of which, and his solution is perfectly valid for Bash users. However, to be more portable, command -v shall be used instead:

$ command -v foo >/dev/null 2>&1 || { echo "I require foo but it's not installed.  Aborting." >&2; exit 1; }

Command command is POSIX compliant. See here for its specification: command - execute a simple command

Note: type is POSIX compliant, but type -P is not.

Answer 5

To mimic Bash's type -P cmd, we can use the POSIX compliant env -i type cmd 1>/dev/null 2>&1.

man env
# "The option '-i' causes env to completely ignore the environment it inherits."
# In other words, there are no aliases or functions to be looked up by the type command.

ls() { echo 'Hello, world!'; }

ls
type ls
env -i type ls

cmd=ls
cmd=lsx
env -i type $cmd 1>/dev/null 2>&1 || { echo "$cmd not found"; exit 1; }

Answer 6

The hash-variant has one pitfall: On the command line you can for example type in

one_folder/process

to have process executed. For this the parent folder of one_folder must be in $PATH. But when you try to hash this command, it will always succeed:

hash one_folder/process; echo $? # will always output '0'