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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesIn a Linux shell script, an if statement runs one command list when a test command exits with status zero and can choose alternate lists with elif and else. Bash uses this form:
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if test-commands; then
consequent-commands
elif more-test-commands; then
more-consequents
else
alternate-consequents
fiThe middle and final branches are optional. The closing keyword is fi, and Bash evaluates conditions by exit status rather than by text printed to the terminal.
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The basic Bash if/elif/else syntax
The reserved words if, then, elif, else and fi delimit the conditional construct. A test can be any command or command list. Put then after the test on the same line with a semicolon, or place it on the following line.
if command; then
commands-when-successful
fi
if command
then
commands-when-successful
fiUse elif for additional tests in sequence. Bash does not use else if as a combined keyword; writing else followed by a nested if is valid, but it requires another matching fi and is usually harder to read.
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if first_test; then
first_action
elif second_test; then
second_action
else
fallback_action
fiHow Bash decides which branch runs
Bash executes the test command list first. Exit status zero is treated as true, so the corresponding command list runs. A nonzero status is treated as false, causing Bash to try the next elif test. If all tests fail and an else branch exists, Bash runs that branch.
This means if checks a command’s status, not whether the command printed output. For example, grep -q can test whether text exists without displaying a match:
if grep -q 'ERROR' application.log; then
printf '%sn' 'Errors were found'
else
printf '%sn' 'No errors were found'
fiThe status returned by the complete conditional is the status of the last command executed in the selected branch. If no condition is true and there is no else, the conditional returns zero because no condition tested true. Do not confuse that result with the status of a test that failed.
Choose the conditional form for your shell
The interpreter named by the shebang determines which syntax is available. Bash-specific constructs are convenient, but a script invoked with /bin/sh must use POSIX-compatible syntax.
| Form | Typical interpreter | Strengths | Important requirement |
|---|---|---|---|
[[ ... ]] |
Bash | Compound conditional syntax for string, numeric and file-attribute tests; ordinary string operands are handled more safely than with the old bracket command. | Do not use it in a script whose interpreter is a shell that does not implement Bash conditionals. |
(( ... )) |
Bash | Readable arithmetic expressions such as score >= 90. |
It is Bash-specific. |
[ ... ] or test |
POSIX sh and Bash |
Portable string, numeric and file tests. | Every operator and operand must be a separate argument, and the closing ] is required. |
Use a Bash shebang when you intend to use [[ ]] or (( )):
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#!/usr/bin/env bashFor a portable script, use:
#!/bin/shTesting strings safely
[[ -z value ]] tests whether a string is empty. The following script accepts a name as its first argument and reports a usage error when it is missing:
#!/usr/bin/env bash
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 the first argument is unset. Quoting the expansion makes the intended single string explicit. In Bash’s [[ ]], ordinary string operands do not undergo the word-splitting and pathname expansion that commonly cause trouble with the single-bracket command, but quoting remains clear and consistent.
For POSIX sh, use separate arguments and quote expansions:
#!/bin/sh
name=${1-}
if [ -n "$name" ]; then
printf '%sn' 'name is set'
else
printf '%sn' 'name is empty'
fiHere -n means the string has nonzero length. The spaces are syntax, not decoration: [ is a command, and the closing ] is its final argument.
Equality and the spacing rule
With the portable form, write:
if [ "$answer" = 'yes' ]; then
printf '%sn' 'Continuing'
fiDo not write [ "$answer"=yes ]. Without spaces, the shell passes a different set of arguments to the bracket command and the test can fail or produce a diagnostic. Quote variables because an empty value or whitespace can otherwise remove or split arguments.
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Comparing numbers
Bash arithmetic conditions use (( ... )) and familiar mathematical operators:
#!/usr/bin/env bash
score=${1:-0}
if (( score >= 90 )); then
printf '%sn' 'excellent'
elif (( score >= 60 )); then
printf '%sn' 'pass'
else
printf '%sn' 'needs improvement'
fiThe first true branch wins, so the highest threshold must come first. The default expression ${1:-0} uses zero when the argument is unset or empty.
In POSIX sh, use the test operators -ge, -gt, -eq, -ne, -lt and -le:
#!/bin/sh
score=${1:-0}
if [ "$score" -ge 90 ]; then
printf '%sn' 'excellent'
elif [ "$score" -ge 60 ]; then
printf '%sn' 'pass'
else
printf '%sn' 'needs improvement'
fiThese operators compare numeric values; do not replace -ge with the string operator >= in a single-bracket POSIX test.
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Checking files and directories
Bash’s compound conditional supports file-attribute tests such as -f for a regular file and -d for a directory:
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#!/usr/bin/env bash
file=${1-}
if [[ -f "$file" ]]; then
printf '%sn' 'regular file'
elif [[ -d "$file" ]]; then
printf '%sn' 'directory'
else
printf '%sn' 'missing or another file type'
fiThe same checks are portable with single brackets:
if [ -f "$file" ]; then
printf '%sn' 'regular file'
elif [ -d "$file" ]; then
printf '%sn' 'directory'
fiKeep the path in a quoted variable. A path containing spaces should still be passed as one argument, and an unset path should not change the number of arguments received by the test command.
Combining conditions and controlling precedence
Within Bash’s [[ ]], combine expressions with && for “and” and || for “or”:
if [[ -n "$name" && "$name" != 'root' ]]; then
printf '%sn' 'A non-root name was supplied'
fiFor portable scripts, separate command tests with shell lists:
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printf '%sn' 'A non-root name was supplied'
fiUse parentheses only when you understand the shell’s parsing rules, and group complex logic into clearly named tests or separate branches. A short, explicit conditional is easier to audit than a dense expression whose precedence is unclear.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Writing several alternatives: elif or case?
An elif chain is appropriate when each branch has a different test, such as numeric ranges or file attributes. When one value must be matched against several literal or pattern alternatives, case is often easier to scan:
case ${1-} in
start)
printf '%sn' 'Starting'
;;
stop)
printf '%sn' 'Stopping'
;;
*)
printf '%sn' 'Usage: script.sh {start|stop}' >&2
exit 2
;;
esaccase is a separate conditional construct, not another spelling of if. Use it when the alternatives are primarily value patterns; use if when each decision involves a command status, range, or different kind of test.
A complete practical script
This Bash example validates an argument, distinguishes a regular file from a directory, and then reports the result. It demonstrates the order in which checks should be made and gives a nonzero status for invalid usage.
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#!/usr/bin/env bash
path=${1-}
if [[ -z "$path" ]]; then
printf '%sn' 'Usage: inspect.sh PATH' >&2
exit 2
elif [[ -f "$path" ]]; then
printf 'Regular file: %sn' "$path"
elif [[ -d "$path" ]]; then
printf 'Directory: %sn' "$path"
else
printf 'Not a regular file or directory: %sn' "$path"
exit 1
fiThe empty-argument check comes first so later file tests never operate on an unintended empty path. The script exits with status 2 for incorrect usage and status 1 for an existing path of another type; successful branches finish with the status returned by their final printf.
Why “syntax error near then” happens
The test line lacks a separator
This is invalid because Bash cannot tell where the test command ends:
if [[ -f "$file" ]] thenUse a semicolon before then or put then on its own line:
if [[ -f "$file" ]]; then
printf '%sn' 'file'
fi
if [[ -f "$file" ]]
then
printf '%sn' 'file'
fiThe closing bracket is missing
Single-bracket tests need both the opening [ and closing ] as separate tokens:
if [ -f "$file" ]; then
printf '%sn' 'file'
fiThe branch keyword is wrong
Use elif for a middle branch. If you write else if, Bash parses it as a new nested conditional, which needs its own then and fi.
The script is running under the wrong shell
A file containing [[ ]] or (( )) can fail when invoked through a POSIX shell. Make the shebang match the syntax and invoke the script directly, or rewrite the tests using [ ... ] for portability.
Quick Recap
A reliable debugging workflow
- Check parsing without executing commands. Run
bash -n script.sh. This catches unmatchedfi, malformed brackets, and misplaced reserved words without running the script body. - Trace the executed branches. Run
bash -x script.sh, or placeset -xbefore the failing section. The trace shows expanded commands and makes it clear which test returned false. - Lint the script. ShellCheck performs static analysis for sh and Bash scripts and flags common syntax, quoting, and portability problems.
- Confirm the interpreter. Inspect the first line and how the script is launched. A Bash script run explicitly with another shell does not get Bash’s conditional syntax.
- Reduce the test. Replace a complicated condition temporarily with a simple command such as
trueorfalse, then add tests back one at a time. This separates parsing errors from unexpected command statuses.
Practical checklist
- Close every conditional with
fi. - Use
elif, not a pseudo-keyword such aselse if. - Separate the test command and
thenwith a semicolon or newline. - Keep spaces around every operator and operand in
[ ... ], and include the closing]. - Quote variable expansions in portable tests, especially when a value may be empty or contain whitespace.
- Use
[[ ... ]]and(( ... ))only when Bash is the interpreter. - Remember that status zero means true in an
ifcondition, while nonzero means false. - Choose
casewhen one value is being matched against many literal or pattern alternatives. - Run
bash -n, thenbash -x, and lint with ShellCheck when a conditional behaves unexpectedly.
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