Bash Cheatsheet - Shell Scripting Command Reference

Q: How do I check whether the previous command succeeded, and how do I make my script fail on purpose in Bash?

$? holds the exit status of the immediately preceding command, where 0 means success and non-zero means failure, so copy it into a variable before checking. Use exit 1 to exit with a non-zero status deliberately, while exit 0 signals success. Combined with set -e, the script automatically aborts on any uncaught failure.

Most Bash scripts only need a few patterns done right: quoting and ${VAR:-default} guards, $? and test conditions, for/while loops, and functions instead of copy-paste. This table is the subset you actually reach for while writing or fixing scripts — pair it with `set -euo pipefail` and the script stops being fragile.

SysOps·55 commands·Last updated 2026-09-10

Typical Use Case

Write scripts, process text and logs, batch rename and find files, compose pipelines on servers, and troubleshoot script permission, unexported variables, and unexpanded globs.

Variables 8

NAME="value"

Define variable (no spaces around =)

echo $NAME

Read variable

echo ${NAME}

Read variable (recommended, avoids ambiguity)

readonly NAME="value"

Define readonly variable

unset NAME

Delete variable

${VAR:-default}

Use default value if variable is empty

${VAR:=default}

Assign and use default if variable is empty

${#VAR}

Get string length

Conditionals 7

if [ condition ]; then ... fi

if statement

if [ -f file ]; then ... fi

Check if file exists

if [ $a -eq $b ]; then ... fi

Numeric comparison (-eq -ne -gt -lt)

if [ "$str" = "value" ]; then ... fi

String comparison (= !=)

if [[ $str =~ regex ]]; then ... fi

Regex match (double brackets)

[ -d dir ] && echo "exists"

Short-circuit evaluation

case $var in pattern1) ... ;; pattern2) ... ;; esac

case statement

Loops 6

for i in 1 2 3; do echo $i; done

for loop over a list

for ((i=0; i<10; i++)); do ... done

C-style for loop

while [ condition ]; do ... done

while loop

until [ condition ]; do ... done

until loop (executes while condition is false)

break / continue

Break out of loop / skip iteration

select var in list; do ... done

Interactive selection menu

Functions 7

function name() { ... }

Define a function

name() { ... }

Define function (shorthand)

name arg1 arg2

Call a function

$1 $2 $@

Positional parameters / all parameters

$#

Number of parameters

return 0

Return value (0 means success)

local var="value"

Local variable

String Operations 7

${VAR:offset:length}

Substring extraction

${VAR#pattern}

Remove shortest matching prefix

${VAR##pattern}

Remove longest matching prefix

${VAR%pattern}

Remove shortest matching suffix

${VAR%%pattern}

Remove longest matching suffix

${VAR/pattern/replacement}

Replace first match

${VAR//pattern/replacement}

Replace all matches

File Tests 8

[ -f file ]

File exists and is a regular file

[ -d dir ]

Directory exists

[ -e path ]

Path exists

[ -r file ]

File is readable

[ -w file ]

File is writable

[ -x file ]

File is executable

[ -s file ]

File size is greater than 0

[ file1 -nt file2 ]

file1 is newer than file2

Parameter Expansion & Debugging (REQ-02 add-on) 12

${var:-default}

Use default when var is unset or empty

${var:=default}

Assign default and write back when empty

${var:?message}

Exit with an error message when empty

${var#pattern}

Remove shortest match from the beginning

${var%pattern}

Remove shortest match from the end

${var/pattern/replace}

Replace the first match

${var^^}

Convert variable content to uppercase

${#var}

Get the character length of a variable

set -x

Enable execution trace of expanded commands

bash -n script.sh

Syntax-check a script without running it

shopt -s nullglob

Expand unmatched globs to empty instead of literal

getopts "a:b" opt

Parse short options in a script

Parameter matrix

参数	Effect	Example
`-x`	执行时打印每条命令，便于调试脚本	`bash -x script.sh`
`-e`	命令失败时立即退出脚本	`set -e`
`-u`	使用未定义变量时报错	`set -u`
`-o pipefail`	管道任一环节失败即视为失败	`set -eo pipefail`
`2>`	重定向标准错误	`cmd 2>/dev/null`
`\|`	管道，前一命令输出作为后一输入	`cat log \| grep error \| wc -l`
`$()`	命令替换，捕获输出	`now=$(date +%F)`
`&& / \|\|`	前令成功/失败才执行后令	`make && make install`
`-name / -type`	find 按名/按类型查找	`find . -name "*.log" -mtime +7`
`-i`	忽略大小写（grep 等）	`grep -ri error log/`

Common pitfalls

Symptom脚本报 Permission denied 无法执行。

Cause文件没有可执行位，或脚本首行 shebang 指向不存在的解释器。

Fixchmod +x script.sh；确认首行 #!/usr/bin/env bash 且 bash 路径存在。

Symptom变量在子 shell/脚本中为空。

Cause未用 export 导出，或作用域仅限当前 shell。

Fix用 export VAR=val 让子进程可见；在脚本内统一用 local/全局变量声明。

Symptomfor f in * 通配符没展开，被当成字面量。

Cause目录为空时 nullglob 未开启，* 保持原样传给命令。

Fix开启 shopt -s nullglob；或用 find ... -print0 | xargs -0 处理空目录情况。

Symptom重定向或管道把文件清空了。

Cause命令如 cat file > file 在读取前先截断目标。

Fix重定向到不同文件或临时文件再覆盖；小心 > 与 >> 的方向。

Symptom空格/特殊字符把文件名拆成多个参数。

Cause未对变量加双引号，IFS 按空格/制表符切割。

Fix始终用 "$var" 包裹变量；处理文件名用 while IFS= read -r line 与 find -print0。

Symptomrm 误删了不该删的文件。

Cause通配符匹配过宽，或路径变量为空。

Fixrm 前先 echo 看匹配结果；危险操作加 -i 确认；避免在变量为空时 rm -rf "$dir"/*。

Troubleshooting

1调试脚本每一步
bash -x script.sh
打印实际执行的命令与变量展开，定位逻辑错误。
2确认脚本语法无误
bash -n script.sh
只做语法检查不执行，适合上线前校验。
3定位最近修改的大文件
find /var -type f -size +100M -mtime -1
排查磁盘突增时快速定位可疑文件。
4统计日志中错误出现次数
grep -ci error /var/log/app.log
快速评估错误规模，结合 tail 看最新样例。

Command Examples

Default a possibly-unset variable

NAME=${NAME:-lzh}
echo "$NAME"

${var:-default} 在变量未设或为空时取 default;改用 ${var-default} 只在完全未设时才取。

Output

lzh   # NAME 未设时输出默认值;已设则输出原值

Test whether a file exists

if [[ -f /etc/nginx.conf ]]; then
  echo exists
else
  echo missing
fi

用 [[ ]] 而非 [ ],支持 && 与 || 且不会错误分词;变量务必用双引号包裹,避免空值时语法报错。

Output

exists

Resolve the directory of the running script

DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
echo "$DIR"

用 BASH_SOURCE[0] 而非 $0,这样即使脚本被 source 引入,拿到的也是其真实所在目录。

Output

/home/deploy/scripts

Common Pitfalls

Prefer [[ ]] and (( )) for tests and arithmetic; [ ] trips on empty variables or strings containing wildcards.
Quoting variables is a hard rule: for i in $LIST or rm $f without quotes split words or delete the wrong files when spaces or globs are involved.
set -e means fail fast: a failing line exits instead of continuing. Enable it in important scripts to avoid cascading failures.
Variables in functions are global by default; forgetting local pollutes globals and setups with the same name are easy to hit.
Both rm -rf and > redirection really delete or overwrite; echo the command as a preview or guard before running against production data.

Tips

No spaces around = in variable assignment, or it will be interpreted as a command.
Always quote variables ("$VAR") to avoid word splitting and special character issues.
[ ] is syntactic sugar for the test command; [[ ]] is a Bash extension with more features like regex matching.

FAQ

What do set -euo pipefail do respectively at the top of a Bash script?

set -e exits on an error, but a failing command does not trigger the exit when it appears in a condition such as if/while/&&/|| or in the middle of a pipeline; -u makes referencing an undefined variable an error; -o pipefail makes a pipeline return the rightmost non-zero exit code among its commands rather than only the last one. Combined they surface errors early and are the standard for robust scripts.

Why does a variable expand inside double quotes but not single quotes in Bash?

Inside double quotes, $variables, $(commands), and backticks are expanded normally, whereas single quotes treat everything literally with no expansion, which suits paths, regexes, or text that must stay untouched. Always wrap a variable holding a path you want to keep intact in double quotes, or word splitting will break it apart.

What is the difference between $@ and $* in Bash?

Without quoting both are equivalent and split the arguments on whitespace. The decisive difference appears when quoted: the @ form preserves each argument as a separate word, whereas the * form joins all arguments into one string. Prefer the quoted at-sign form when iterating over positional parameters.

How do I read a file line by line in Bash robustly?

Use while IFS= read -r line; do ...; done < file. IFS= prevents stripping leading and trailing whitespace, and -r keeps backslashes literal. Do not use for line in $(cat file), which splits the file into words on whitespace and loses line structure.

How do I check whether the previous command succeeded, and how do I make my script fail on purpose in Bash?

$? holds the exit status of the immediately preceding command, where 0 means success and non-zero means failure, so copy it into a variable before checking. Use exit 1 to exit with a non-zero status deliberately, while exit 0 signals success. Combined with set -e, the script automatically aborts on any uncaught failure.

Official References

Each command links to its official documentation below, so you can verify the latest usage and read deeper.

Linux man-pages

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Publicly updated on Sep 10, 2026, continuously proofread against official docs.

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Typical Use Case

Variables 8

Conditionals 7

Loops 6

Functions 7

String Operations 7

File Tests 8

Parameter Expansion & Debugging (REQ-02 add-on) 12

Parameter matrix

Common pitfalls

Troubleshooting

Command Examples

Default a possibly-unset variable

Test whether a file exists

Resolve the directory of the running script

Common Pitfalls

Tips

FAQ

Related Cheatsheets

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Official References

Maintained by LaoHand

Contact Us