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Bash if Statements: Conditions, test and case

Write conditions in Bash with if, [[ ]] and test: compare strings and numbers, check files, combine tests with && and ||, and use case statements.

Intermediate8 min readUpdated
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if#

Terminal
marks=72
if [[ $marks -ge 40 ]]; then
  echo "Pass"
else
  echo "Fail"
fi
Output
Pass

if ends with fi. The spaces inside [[ ]] are required.

elif adds more branches:

Terminal
marks=85
if [[ $marks -ge 75 ]]; then
  echo "Distinction"
elif [[ $marks -ge 40 ]]; then
  echo "Pass"
else
  echo "Fail"
fi
Output
Distinction

Comparing numbers#

OperatorMeaning
-eq, -neEqual, not equal
-gt, -geGreater than, or equal
-lt, -leLess than, or equal

With (( )), you can use normal symbols:

Terminal
users=12
if (( users > 10 )); then
  echo "Busy"
fi
Output
Busy

Comparing text#

TestTrue when
"$a" == "$b"The text is equal
"$a" != "$b"The text differs
-z "$a"Empty
-n "$a"Not empty
"$a" == prod*Matches a pattern
"$a" =~ ^[0-9]+$Matches a regular expression
Terminal
env_name="production"
if [[ "$env_name" == prod* ]]; then
  echo "Careful: live system"
fi
input="1042"
if [[ "$input" =~ ^[0-9]+$ ]]; then
  echo "A number"
fi
Output
Careful: live system
A number

Testing files#

TestTrue when
-e pathIt exists
-f pathIt is a file
-d pathIt is a folder
-s pathIt is not empty
-r, -w, -xIt is readable, writable, executable
Terminal
if [[ -f /etc/hostname ]]; then
  echo "File exists"
fi
if [[ ! -d /backup ]]; then
  echo "No backup folder"
fi
Output
File exists
No backup folder

! means "not".

Combining#

Use && for "and", and || for "or":

Terminal
age=20
member="yes"
if [[ $age -ge 18 && "$member" == "yes" ]]; then
  echo "Allowed"
fi
Output
Allowed

Commands as conditions#

if really checks a command's exit code, where 0 means success. So any command can be the condition:

Terminal
if grep -q root /etc/passwd; then
  echo "root exists"
fi
if ! ping -c 1 -W 1 192.0.2.1 > /dev/null 2>&1; then
  echo "Host is not answering"
fi
Output
root exists
Host is not answering

Short forms#

&& runs the next command if the first succeeds, and || runs it if the first fails:

Terminal
mkdir -p logs && echo "Folder ready"
ls /nothing 2> /dev/null || echo "Could not list it"
Output
Folder ready
Could not list it

[[ ]] or [ ]?#

[ ] is the older form, and works in any POSIX shell. [[ ]] is Bash's improved version, with pattern matching, &&, and safer handling of empty variables. Use [[ ]] in Bash scripts. If a script starts with #!/bin/sh, use [ ].

case#

case is cleaner than a long if when one value has many options:

Terminal
action="restart"
case "$action" in
  start)
    echo "Starting"
    ;;
  stop)
    echo "Stopping"
    ;;
  restart|reload)
    echo "Restarting"
    ;;
  *)
    echo "Unknown action"
    ;;
esac
Output
Restarting

*) catches everything else, and ;; ends each branch.

Common mistakes#

Practice#

Practice exercise

Write a script that takes a disk usage number and prints OK below 80, WARNING from 80 to 89, and CRITICAL from 90. Test it with 72, 85 and 95 using a loop.

Show solution
Terminal
for used in 72 85 95; do
  if (( used >= 90 )); then
    echo "$used CRITICAL"
  elif (( used >= 80 )); then
    echo "$used WARNING"
  else
    echo "$used OK"
  fi
done
Output
72 OK
85 WARNING
95 CRITICAL

Key takeaway#

Next: Loops.

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