Correct way to check for a command line flag in bash

Question

In the middle of a script, I want to check if a given flag was passed on the command line. The following does what I want but seems ugly:

if echo $* | grep -e "--flag" -q
then
  echo ">>>> Running with flag"
else
  echo ">>>> Running without flag"
fi

Is there a better way?

Note: I explicitly don't want to list all the flags in a switch/getopt. (In this case any such things would become half or more of the full script. Also the bodies of the if just set a set of vars)

Answer 1

An alternative to what you're doing:

if [[ $* == *--flag* ]]

See also BashFAQ/035.

Note: This will also match --flags-off since it's a simple substring check.

Answer 2

I typically see this done with a case statement. Here's an excerpt from the git-repack script:

while test $# != 0
do
    case "$1" in
    -n) no_update_info=t ;;
    -a) all_into_one=t ;;
    -A) all_into_one=t
        unpack_unreachable=--unpack-unreachable ;;
    -d) remove_redundant=t ;;
    -q) GIT_QUIET=t ;;
    -f) no_reuse=--no-reuse-object ;;
    -l) local=--local ;;
    --max-pack-size|--window|--window-memory|--depth)
        extra="$extra $1=$2"; shift ;;
    --) shift; break;;
    *)  usage ;;
    esac
    shift
done

Note that this allows you to check for both short and long flags. Other options are built up using the extra variable in this case.

Answer 3

you can take the simple approach, and iterate over the arguments to test each of them for equality with a given parameter (e.g. -t).

put it into a function:

has_param() {
    local term="$1"
    shift
    for arg; do
        if [[ $arg == "$term" ]]; then
            return 0
        fi
    done
    return 1
}

… and use it as a predicate in test expressions:

if has_param '-t' "$@"; then
    echo "yay!"
fi

if ! has_param '-t' "$1" "$2" "$wat"; then
    echo "nay..."
fi

if you want to reject empty arguments, add an exit point at the top of the loop body:

for arg; do
    if [[ -z "$arg" ]]; then
        return 2
    fi
    # ...

this is very readable, and will not give you false positives, like pattern matching or regex matching will.
it will also allow placing flags at arbitrary positions, for example, you can put -h at the end of the command line (not going into whether it's good or bad).

---

but, the more i thought about it, the more something bothered me.

with a function, you can take any implementation (e.g. getopts), and reuse it. encapsulation rulez!
but even with commands, this strength can become a flaw. if you'll be using it again and again, you'll be parsing all the arguments each time.

my tendency is to favor reuse, but i have to be aware of the implications. the opposed approach would be to parse these arguments once at the script top, as you dreaded, and avoid the repeated parsing.
you can still encapsulate that switch case, which can be as big as you decide (you don't have to list all the options).

Answer 4

You can use the getopt keyword in bash.

From http://aplawrence.com/Unix/getopts.html:

getopt

This is a standalone executable that has been around a long time. Older versions lack the ability to handle quoted arguments (foo a "this won't work" c) and the versions that can, do so clumsily. If you are running a recent Linux version, your "getopt" can do that; SCO OSR5, Mac OS X 10.2.6 and FreeBSD 4.4 has an older version that does not.

The simple use of "getopt" is shown in this mini-script:

#!/bin/bash
echo "Before getopt"
for i
do
  echo $i
done
args=`getopt abc:d $*`
set -- $args
echo "After getopt"
for i
do
  echo "-->$i"
done

Answer 5

I've made small changes to the answer of Eliran Malka:

This function can evaluate different parameter synonyms, like "-q" and "--quick". Also, it does not use return 0/1 but an echo to return a non-null value when the parameter is found:

function has_param() {
    local terms="$1"
    shift

    for term in $terms; do
        for arg; do
            if [[ $arg == "$term" ]]; then
                echo "yes"
            fi
        done
    done
}

# Same usage:

# Assign result to a variable.
FLAG_QUICK=$(has_param "-q --quick" "$@")  # "yes" or ""

# Test in a condition using the nonzero-length-test to detect "yes" response.
if [[ -n $(has_param "-h --help" "$@") ]]; then; 
    echo "Need help?"
fi

# Check, is a flag is NOT set by using the zero-length test.
if [[ -z $(has_param "-f --flag" "$@") ]]; then
    echo "FLAG NOT SET"
fi

Answer 6

Not an alternative, but an improvement, though.

if echo $* | grep -e "\b--flag\b" -q

Looking for word boundaries will make sure to really get the option --flag and neither --flagstaff nor --not-really--flag

Answer 7

The modification of Dennis Williamson's answer with additional example for a argument in the short form.

if [[ \ $*\  == *\ --flag\ * ]] || [[ \ $*\  == *\ -f\ * ]]

It solves the problem of false positive matching --flags-off and even --another--flag (more popular such case for an one-dashed arguments: --one-more-flag for *-f*).

\ (backslash + space) means space for expressions inside [[ ]]. Putting spaces around $* allows to be sure that the arguments contacts neither line's start nor line's end, they contacts only spaces. And now the target flag surrounded by spaces can be searched in the line with arguments.

Answer 8

if [ "$1" == "-n" ]; then
    echo "Flag set";
fi