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In a Bash script, an if statement runs a command list and treats exit status 0 as true. Use elif for additional tests, else for the fallback, and close the construct with fi:
if test-commands; then
consequent-commands
elif more-test-commands; then
more-consequents
else
alternate-consequents
fiThe same structure works with string, numeric, and file tests. The important portability choice is whether the script runs under Bash (where [[ ... ]] and (( ... )) are available) or under a POSIX shell such as /bin/sh (where use [ ... ] or test).
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What an if statement actually tests
Shell conditionals do not test whether a command printed a particular word. They execute the test command list and inspect its exit status. Status 0 means success and is treated as true; any nonzero status is false. Bash executes the first true branch, skips the remaining branches, and returns the status of the last command it ran. If no condition succeeds and there is no else, the compound command returns zero.
This design lets you test ordinary commands directly:
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if grep -q '^admin:' /etc/passwd; then
printf '%sn' 'An admin account exists'
else
printf '%sn' 'No admin account was found'
fiHere, grep -q supplies the condition through its exit status. The output of the command is irrelevant to the decision.
Basic Bash syntax
The reserved words that delimit the construct are if, then, elif, else, and fi. The elif and else clauses are optional, and you can have multiple elif clauses.
Putting then on the same line
if [[ -f "$1" ]]; then
printf '%sn' 'Regular file'
fiA semicolon separates the test command from then when both are on one line. Without that separator, Bash interprets the line incorrectly and reports a syntax error.
Putting then on the next line
if [[ -f "$1" ]]
then
printf '%sn' 'Regular file'
fiA newline terminates the test command, so no semicolon is needed in this form. Choose one style and apply it consistently.
Using several branches
if command-a; then
printf '%sn' 'A succeeded'
elif command-b; then
printf '%sn' 'B succeeded'
else
printf '%sn' 'Both failed'
fiBash evaluates the tests from top to bottom. Once a test returns zero, its commands run and later tests are not evaluated.
A complete argument-checking example
This script demonstrates a common pattern: capture an optional argument, test it safely, and return a useful error status when it is missing.
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name=${1-}
if [[ -z "$name" ]]; then
printf '%sn' 'Usage: script.sh NAME' >&2
exit 2
else
printf 'Hello, %sn' "$name"
fi${1-} supplies an empty value when no first argument was provided. The -z test is true for an empty string. The diagnostic is redirected to standard error, while a valid name is printed on standard output. Quoting "$name" preserves spaces and prevents word splitting.
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Choosing a condition syntax
There are three commonly encountered forms. Their capabilities overlap, but their portability and parsing rules differ.
| Form | Best use | Interpreter requirement | Important rules |
|---|---|---|---|
[[ ... ]] |
Readable Bash string, numeric, file, and pattern conditions | Bash | Use as a Bash compound conditional; quote ordinary variable expansions |
[ ... ] |
Portable single-condition tests | POSIX shells and Bash | [ is a command-like token; every argument needs separation and the closing ] is required |
test ... |
The same portable tests without bracket notation | POSIX shells and Bash | Operators and operands remain separate arguments |
(( ... )) |
Arithmetic comparisons and calculations | Bash | Use Bash as the interpreter; the expression’s arithmetic result determines status |
If the file begins with #!/bin/sh, write POSIX-compatible tests. A Bash shebang such as #!/usr/bin/env bash is required before using [[ ... ]] or (( ... )).
String comparisons
Using Bash [[ ... ]]
[[ ... ]] is Bash’s compound conditional command. It supports unary and binary conditional expressions and is generally easier to read for ordinary string operands.
name=${1-}
if [[ -z "$name" ]]; then
printf '%sn' 'Name is empty'
elif [[ "$name" == 'root' ]]; then
printf '%sn' 'The root account name was supplied'
else
printf 'Name: %sn' "$name"
fiUseful unary string checks include -z for an empty string and -n for a non-empty string. Binary operators compare values, for example == and !=.
Using POSIX [ ... ]
#!/bin/sh
name=${1-}
if [ -n "$name" ]; then
printf '%sn' 'name is set'
else
printf '%sn' 'name is empty'
fiWith bracket syntax, spacing is part of the grammar. Write:
if [ "$x" = yes ]; then
printf '%sn' 'enabled'
fiDo not write [ "$x"=yes ]. In the latter form, the shell passes a different set of arguments to the bracket command, so the intended comparison is not performed.
Why quoting matters
Always quote variable expansions in portable bracket tests. An empty expansion can remove an argument, and a value containing whitespace can become several arguments. Both situations change how the test command parses its operands.
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if [ "$answer" = yes ]; then
printf '%sn' 'Continuing'
fiInside [[ ... ]], ordinary quoted expansions are still the clearest and safest habit. Keep pattern syntax intentional rather than relying on accidental expansion behavior.
Numeric comparisons
Bash arithmetic with (( ... ))
For Bash scripts, arithmetic evaluation gives a concise way to branch on numbers:
score=${1:-0}
if (( score >= 90 )); then
printf '%sn' 'excellent'
elif (( score >= 60 )); then
printf '%sn' 'pass'
else
printf '%sn' 'needs improvement'
fiThe arithmetic expression itself determines the status: a nonzero result is true and zero is false. The variable name can be used directly inside (( ... )); you do not need the $ prefix.
POSIX numeric operators
In [ ... ] or test, use numeric operators rather than string operators:
#!/bin/sh
score=${1:-0}
if [ "$score" -ge 90 ]; then
printf '%sn' 'excellent'
elif [ "$score" -ge 60 ]; then
printf '%sn' 'pass'
else
printf '%sn' 'needs improvement'
fiCommon numeric operators include -eq (equal), -ne (not equal), -lt (less than), -le (less than or equal), -gt (greater than), and -ge (greater than or equal). Keep spaces around every operand and operator.
Checking files and directories
File-attribute tests let a script choose an action based on what a path refers to:
#!/usr/bin/env bash
file=${1-}
if [[ -f "$file" ]]; then
printf '%sn' 'regular file'
elif [[ -d "$file" ]]; then
printf '%sn' 'directory'
else
printf '%sn' 'missing or another file type'
fi-f succeeds for a regular file and -d succeeds for a directory. The final branch also covers a path that does not exist and file types not matched by those two tests. Add other file-attribute expressions when your script needs them, while keeping each branch’s purpose explicit.
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Combining conditions
When one branch depends on more than one fact, combine expressions in the conditional form supported by your shell. In Bash, [[ ... ]] can express compound conditions directly:
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if [[ -f "$config" && -r "$config" ]]; then
printf '%sn' 'Readable configuration file'
else
printf '%sn' 'Configuration is missing or unreadable'
fiFor POSIX scripts, separate tests with shell operators and group them deliberately:
if [ -f "$config" ] && [ -r "$config" ]; then
printf '%sn' 'Readable configuration file'
fiKeeping each bracket expression complete makes argument boundaries visible and avoids malformed tests.
When case is clearer than elif
Use if when conditions are ranges, file attributes, command results, or unrelated predicates. Use case when one value must be matched against several literal or pattern alternatives. A long chain that repeatedly compares the same variable is often easier to scan as a case construct:
case ${1-} in
start)
printf '%sn' 'Starting'
;;
stop)
printf '%sn' 'Stopping'
;;
restart)
printf '%sn' 'Restarting'
;;
*)
printf '%sn' 'Usage: script.sh {start|stop|restart}' >&2
exit 2
;;
esaccase is a separate conditional construct, not another spelling of elif. Choose it when the alternatives are values or patterns rather than progressively tested conditions.
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The status of the entire if construct is the status of the last command executed in the selected branch. That detail matters when a function or a larger script relies on the conditional’s result.
if [[ -f "$path" ]]; then
printf '%sn' 'Found'
return 0
else
printf '%sn' 'Not found' >&2
return 1
fiAt top level, use exit instead of return. If you want the conditional itself to communicate success or failure, make the final command in each branch an intentional status-producing command.
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Why “syntax error near then” happens
The test command was not terminated
On a one-line condition, then must follow a semicolon:
if [[ "$mode" == 'safe' ]]; then
printf '%sn' 'Safe mode'
fiAlternatively, place then on the next line. A missing semicolon or newline leaves Bash unable to tell where the test command ends.
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An if block closes with fi. The middle branch keyword is elif, not else if:
if condition-a; then
command-a
elif condition-b; then
command-b
else
command-c
fiNested if statements are valid, but each nested construct needs its own matching fi. Use nesting only when a second decision genuinely belongs inside the first branch.
Bracket spacing is missing
Because [ is command-like syntax, these tokens must be separate:
if [ "$x" = yes ]; then
printf '%sn' 'yes'
fiThe opening bracket, variable, operator, comparison value, and closing bracket are individual arguments. Missing any required space changes the command that the shell executes.
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The script is running under the wrong shell
A script containing [[ ... ]] or (( ... )) can fail when invoked by a POSIX shell. Make the shebang match the syntax, then invoke the script in a way that honors it:
#!/usr/bin/env bashIf the script must remain /bin/sh-portable, replace Bash-only constructs with [ ... ], test, and POSIX arithmetic techniques.
A practical debugging workflow
- Check parsing first. Run
bash -n script.sh. Bash reads the file without executing its commands, so this isolates syntax errors. - Trace execution. Run
bash -x script.sh, or placeset -xaround the failing section. The trace shows expanded commands and the branch that actually ran. - Verify the interpreter. Read the shebang and compare it with the constructs in the file. A Bash-only conditional cannot be validated by assuming every
shimplementation behaves like Bash. - Lint the script. Run ShellCheck against the file. Its static analysis flags common shell syntax errors, quoting mistakes, and portability pitfalls that are easy to overlook during manual review.
- Test boundary inputs. Try no argument, an empty value, whitespace in a value, a nonexistent path, a directory where a file is expected, and values exactly on numeric thresholds.
Patterns that prevent common mistakes
- Put a Bash shebang at the top when using
[[ ... ]]or(( ... )). - Quote variable expansions in
[ ... ]tests and in ordinary string comparisons. - Use
eliffor another branch at the same decision level; reserve nestedifblocks for dependent decisions. - Keep one complete condition per line when a test becomes difficult to read.
- Use numeric operators for numbers; do not substitute string equality for a numeric range test.
- Use
casewhen many alternatives all match one value. - Make fallback behavior explicit with
elsewhen silently doing nothing would hide an error. - Return an intentional status from error branches, especially when another command will consume the script’s result.
Quick reference
| Need | Bash example | Portable form |
|---|---|---|
| Non-empty string | [[ -n "$value" ]] |
[ -n "$value" ] |
| Equal strings | [[ "$a" == "$b" ]] |
[ "$a" = "$b" ] |
| Numeric threshold | (( count >= 10 )) |
[ "$count" -ge 10 ] |
| Regular file | [[ -f "$path" ]] |
[ -f "$path" ] |
| Directory | [[ -d "$path" ]] |
[ -d "$path" ] |
Remember the four structural rules that prevent most errors: status zero is true, elif adds a same-level branch, fi closes the block, and bracket tests require exact spacing. Once those rules are in place, select [[ ... ]] and (( ... )) for Bash-specific scripts, or [ ... ] and test when portability to /bin/sh matters.
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