Bash - if statements

Here is the truth about if in Bash: it does not test conditions. It runs a command and looks at whether that command succeeded. [ is just a command (with a funny name) that succeeds or fails. Once you see that, every strange-looking Bash if makes sense.

What it is, in plain words

The syntax is:

if COMMANDS; then
  ...
elif COMMANDS; then
  ...
else
  ...
fi

Bash runs the COMMANDS after if. If the last one exits with status 0 (success), the then branch runs. Any other status (1 to 255) means failure, and Bash moves on to elif or else. Note the shell’s convention is the reverse of C: 0 is true.

Three kinds of commands are commonly used as tests:

[ ... ], also called test: the portable POSIX test command. Spaces around the brackets are required because [ is a command name and ] is its last argument.
[[ ... ]]: a Bash keyword with safer rules. No word splitting inside, so quoting matters less; supports &&, ||, pattern matching with == and regexes with =~.
(( ... )): arithmetic. Succeeds if the expression is non-zero: (( count > 10 )).

And any ordinary command works: if grep -q root /etc/passwd; then ....

Why it matters

Shell scripts are mostly decisions: does this file exist, did the download work, is this the right user. Getting if wrong produces scripts that silently take the wrong branch, which is worse than a crash. And knowing that if checks exit status lets you write tests directly against real commands instead of capturing output and comparing strings.

An everyday analogy: a bouncer with a phone

The bouncer (if) does not judge anyone himself. He phones a checker (the command), who answers only “yes” (0) or “no” (anything else). The bouncer lets you in or sends you to the next door (elif) based on that one answer. [ is just the checker who knows how to compare numbers and look at files.

A hands-on example

#!/usr/bin/env bash
# backup-check.sh  - decisions four ways
set -u

file=${1:-$HOME/notes.org}

# 1. Test a file with [[ ]]
if [[ ! -f $file ]]; then
  echo "No such file: $file" >&2
  exit 1
fi

# 2. Test a command's success directly
if ping -c1 -W2 backup.local &>/dev/null; then
  echo "Backup host is up"
else
  echo "Backup host unreachable, will retry later"
fi

# 3. Arithmetic with (( ))
size=$(stat -c %s -- "$file")
if (( size > 1024 * 1024 )); then
  echo "$file is over 1 MiB"
fi

# 4. Patterns and regexes with [[ ]]
if [[ $file == *.org ]]; then
  echo "An Org file"
elif [[ $file =~ \.(md|markdown)$ ]]; then
  echo "Markdown: extension ${BASH_REMATCH[1]}"
fi

# Short form for a single action
[[ -w $file ]] || echo "Warning: $file is read-only"

Check the exit status of anything yourself with echo $? straight after it runs.

Common misconceptions

"[$# -ne 1] is fine." The example lower down this page has exactly this bug. Without spaces, Bash looks for a command literally named [1 (or similar) and fails. It must be [ $# -ne 1 ].
"= and -eq are the same." = compares strings, -eq compares integers. [ 010 = 10 ] is false, [ 010 -eq 10 ] is true.
“Quoting doesn’t matter in [ ].” If $AGE is empty, [ $AGE -lt 13 ] becomes [ -lt 13 ], a syntax error. Quote ("$AGE") or use [[ ]].
"[[ ]] works everywhere." It is Bash (and zsh, ksh). In #!/bin/sh scripts, which on Void and Debian run dash, use [ ].
"$(whoami) = 'root' is the way to check for root." Comparing $EUID (or "$(id -u)") with 0 is faster and does not depend on the account’s name.

Connections

Bash case statements, Using Variables in Bash, Reserved Words, Vikid’s Bash Cookbook.
C Syntax and JavaScript Control Flow – where if tests values, not commands.
Linux Tools, awk.

Examples

if [ condition ]; then # you can use [[ ]] also
  your code
fi

if [[ "${UID}" -eq 0 ]]
then
  echo 'You are root'
else
  echo 'You are not root'
fi

#!/bin/bash

if [ $(whoami) = 'root' ]; then
	echo "You are root"
fi

In one line:

if [ $(whoami) = 'root' ]; then echo "root"; else echo "not root"; fi

#!/bin/bash

if [ $(whoami) = 'root' ]; then
	echo "You are root"
else
	echo "You are not root"
fi

#!/bin/bash

AGE=$1

if [ $AGE -lt 13 ]; then
	echo "You are a kid."
elif [ $AGE -lt 20 ]; then
	echo "You are a teenager."
elif [ $AGE -lt 65 ]; then
	echo "You are an adult."
else
	echo "You are an elder."
fi

#!/bin/bash
echo Script name: $0
echo $# arguments
if [$# -ne 1];
    then echo "illegal number of parameters"
fi

$a -lt $b	        $a < $b
$a -gt $b	        $a > $b
$a -le $b	        $a <= $b
$a -ge $b	        $a >= $b
$a -eq $b	        $a is equal to $b
$a -ne $b	        $a is not equal to $b
-e $FILE	        $FILE exists
-d $FILE	        $FILE exists and is a directory.
-f $FILE	        $FILE exists and is a regular file.
-L $FILE	        $FILE exists and is a soft link.
$STRING1 = $STRING2	$STRING1 is equal to $STRING2
$STRING1 != $STRING2	$STRING1 is not equal to $STRING2
-z $STRING1	        $STRING1 is empty