Wolfgang Resch - Notes

Short circuit evaluation of conditionals in bash

April 24, 2014

In bash an if statement can often be replaced by short circuit evaluation of a conditional. For example, to check for the existence of a file you could use either

if  [[ -e "$filename" ]]; then
    echo "exists"


[[ -e "$filename" ]] && echo "exists"

What happens in this case is that bash first executes the command/expression on the left of the logical and (&&). If that expression has a non-zero return value (i.e. it failed), then there is no need to evaluate the right side of the && because the overall exit status is already known to be non-zero since both sides have to return a success indicator for a logical and to return success.

The logical or (||) can be used similarly to only execute a command if a test fails:

[[ -e "$filename" ]] || echo "could not find file"

The way this works is by again first evaluating the left side. If the left side is successful, then the logical or is already known to be true and there is no need to execute the right side. The expression results in a 0 exit status. If the left side has a non-zero exit status, then it is still necessary to evaluate the right side to determine the exit status of the whole expression. These two can be combined:

[[ -e "$filename" ]] && echo "file exists" || echo "file does not exist"

These expressions can be useful for easy error checking/cleanup in shell scripts. For example

#! /bin/bash
set -o pipefail
function fail {
    rm -f ${tmp}*
    exit 1
module load bedops || fail
tmp=$(mktemp temp/XXXX)
bedmap --echo --count --delim "\t" $superE $hs > $tmp || fail
bedmap --echo --count --delim "\t" $tmp $foot > ${tmp}.2 || fail
bedmap --echo --indicator --delim "\t" ${tmp}.2 $rpa > ${tmp}.3 || fail
bedmap --echo --echo-map-id --delim "\t" --multidelim "," ${tmp}.3 $motif > ${tmp}.4 \
    && mv ${tmp}.4 $outf \
    || fail
rm -f ${tmp}*

Important caveat: in the combined expression, if the middle command has a non-zero exit status, then both the middle and the rightmost command end up getting executed.