ShellCheck - A shell script static analysis tool

ShellCheck is a GPLv3 tool that gives warnings and suggestions for bash/sh shell scripts:

The goals of ShellCheck are

• To point out and clarify typical beginner's syntax issues that cause a shell to give cryptic error messages.

• To point out and clarify typical intermediate level semantic problems that cause a shell to behave strangely and counter-intuitively.

• To point out subtle caveats, corner cases and pitfalls that may cause an advanced user's otherwise working script to fail under future circumstances.

See the gallery of bad code for examples of what ShellCheck can help you identify!

Table of Contents

• How to use
  • On the web
  • From your terminal
  • In your editor
  • In your build or test suites
• Installing
• Compiling from source
  • Installing Cabal
  • Compiling ShellCheck
  • Running tests
• Gallery of bad code
  • Quoting
  • Conditionals
  • Frequently misused commands
  • Common beginner's mistakes
  • Style
  • Data and typing errors
  • Robustness
  • Portability
  • Miscellaneous
• Testimonials
• Ignoring issues
• Reporting bugs
• Contributing
• Copyright
• Other Resources

How to use

There are a number of ways to use ShellCheck!

On the web

Paste a shell script on https://www.shellcheck.net for instant feedback.

ShellCheck.net is always synchronized to the latest git commit, and is the easiest way to give ShellCheck a go. Tell your friends!

From your terminal

Run shellcheck yourscript in your terminal for instant output, as seen above.

In your editor

You can see ShellCheck suggestions directly in a variety of editors.

• Vim, through ALE, Neomake, or Syntastic:

.

• Emacs, through Flycheck or Flymake:

.

• Sublime, through SublimeLinter.

• Pulsar Edit (former Atom), through linter-shellcheck-pulsar.

• VSCode, through vscode-shellcheck.

• Most other editors, through GCC error compatibility.

In your build or test suites

While ShellCheck is mostly intended for interactive use, it can easily be added to builds or test suites. It makes canonical use of exit codes, so you can just add a shellcheck command as part of the process.

For example, in a Makefile:

check-scripts: # Fail if any of these files have warnings shellcheck myscripts/*.sh 

or in a Travis CI .travis.yml file:

script: # Fail if any of these files have warnings - shellcheck myscripts/*.sh 

Services and platforms that have ShellCheck pre-installed and ready to use:

• Travis CI
• Codacy
• Code Climate
• Code Factor
• CircleCI via the ShellCheck Orb
• Github (only Linux)
• Trunk Check (universal linter; allows you to explicitly version your shellcheck install) via the shellcheck plugin

Most other services, including GitLab, let you install ShellCheck yourself, either through the system's package manager (see Installing), or by downloading and unpacking a binary release.

It's a good idea to manually install a specific ShellCheck version regardless. This avoids any surprise build breaks when a new version with new warnings is published.

For customized filtering or reporting, ShellCheck can output simple JSON, CheckStyle compatible XML, GCC compatible warnings as well as human readable text (with or without ANSI colors). See the Integration wiki page for more documentation.

Installing

The easiest way to install ShellCheck locally is through your package manager.

On systems with Cabal (installs to ~/.cabal/bin):

cabal update cabal install ShellCheck 

On systems with Stack (installs to ~/.local/bin):

stack update stack install ShellCheck 

On Debian based distros:

sudo apt install shellcheck 

On Arch Linux based distros:

pacman -S shellcheck 

or get the dependency free shellcheck-bin from the AUR.

On Gentoo based distros:

emerge --ask shellcheck 

On EPEL based distros:

sudo yum -y install epel-release sudo yum install ShellCheck 

On Fedora based distros:

dnf install ShellCheck 

On FreeBSD:

pkg install hs-ShellCheck 

On macOS (OS X) with Homebrew:

brew install shellcheck 

Or with MacPorts:

sudo port install shellcheck 

On OpenBSD:

pkg_add shellcheck 

On openSUSE

zypper in ShellCheck 

Or use OneClickInstall - https://software.opensuse.org/package/ShellCheck

On Solus:

eopkg install shellcheck 

On Windows (via chocolatey):

C:\> choco install shellcheck 

Or Windows (via winget):

C:\> winget install --id koalaman.shellcheck 

Or Windows (via scoop):

C:\> scoop install shellcheck 

From conda-forge:

conda install -c conda-forge shellcheck 

From Snap Store:

snap install --channel=edge shellcheck 

From Docker Hub:

docker run --rm -v "$PWD:/mnt" koalaman/shellcheck:stable myscript # Or :v0.4.7 for that version, or :latest for daily builds 

or use koalaman/shellcheck-alpine if you want a larger Alpine Linux based image to extend. It works exactly like a regular Alpine image, but has shellcheck preinstalled.

Using the nix package manager:

nix-env -iA nixpkgs.shellcheck 

Alternatively, you can download pre-compiled binaries for the latest release here:

• Linux, x86_64 (statically linked)
• Linux, armv6hf, i.e. Raspberry Pi (statically linked)
• Linux, aarch64 aka ARM64 (statically linked)
• macOS, x86_64
• Windows, x86

or see the GitHub Releases for other releases (including the latest meta-release for daily git builds).

There are currently no official binaries for Apple Silicon, but third party builds are available via ShellCheck for Visual Studio Code.

Distro packages already come with a man page. If you are building from source, it can be installed with:

pandoc -s -f markdown-smart -t man shellcheck.1.md -o shellcheck.1 sudo mv shellcheck.1 /usr/share/man/man1 

pre-commit

To run ShellCheck via pre-commit, add the hook to your .pre-commit-config.yaml:

repos: - repo: https://github.com/koalaman/shellcheck-precommit rev: v0.7.2 hooks: - id: shellcheck # args: ["--severity=warning"] # Optionally only show errors and warnings 

Travis CI

Travis CI has now integrated ShellCheck by default, so you don't need to manually install it.

If you still want to do so in order to upgrade at your leisure or ensure you're using the latest release, follow the steps below to install a binary version.

Installing a pre-compiled binary

The pre-compiled binaries come in tar.xz files. To decompress them, make sure xz is installed. On Debian/Ubuntu/Mint, you can apt install xz-utils. On Redhat/Fedora/CentOS, yum -y install xz.

A simple installer may do something like:

scversion="stable" # or "v0.4.7", or "latest" wget -qO- "https://github.com/koalaman/shellcheck/releases/download/${scversion?}/shellcheck-${scversion?}.linux.x86_64.tar.xz" | tar -xJv cp "shellcheck-${scversion}/shellcheck" /usr/bin/ shellcheck --version 

Compiling from source

This section describes how to build ShellCheck from a source directory. ShellCheck is written in Haskell and requires 2GB of RAM to compile.

Installing Cabal

ShellCheck is built and packaged using Cabal. Install the package cabal-install from your system's package manager (with e.g. apt-get, brew, emerge, yum, or zypper).

On macOS (OS X), you can do a fast install of Cabal using brew, which takes a couple of minutes instead of more than 30 minutes if you try to compile it from source.

$ brew install cabal-install 

On MacPorts, the package is instead called hs-cabal-install, while native Windows users should install the latest version of the Haskell platform from https://www.haskell.org/platform/

Verify that cabal is installed and update its dependency list with

$ cabal update 

Compiling ShellCheck

git clone this repository, and cd to the ShellCheck source directory to build/install:

$ cabal install 

This will compile ShellCheck and install it to your ~/.cabal/bin directory.

Add this directory to your PATH (for bash, add this to your ~/.bashrc):

export PATH="$HOME/.cabal/bin:$PATH" 

Log out and in again, and verify that your PATH is set up correctly:

$ which shellcheck ~/.cabal/bin/shellcheck 

On native Windows, the PATH should already be set up, but the system may use a legacy codepage. In cmd.exe, powershell.exe and Powershell ISE, make sure to use a TrueType font, not a Raster font, and set the active codepage to UTF-8 (65001) with chcp:

chcp 65001 

In Powershell ISE, you may need to additionally update the output encoding:

[Console]::OutputEncoding = [System.Text.Encoding]::UTF8 

Running tests

To run the unit test suite:

$ cabal test 

Gallery of bad code

So what kind of things does ShellCheck look for? Here is an incomplete list of detected issues.

Quoting

ShellCheck can recognize several types of incorrect quoting:

echo $1 # Unquoted variables find . -name *.ogg # Unquoted find/grep patterns rm "~/my file.txt" # Quoted tilde expansion v='--verbose="true"'; cmd $v # Literal quotes in variables for f in "*.ogg" # Incorrectly quoted 'for' loops touch $@ # Unquoted $@ echo 'Don't forget to restart!' # Singlequote closed by apostrophe echo 'Don\'t try this at home' # Attempting to escape ' in '' echo 'Path is $PATH' # Variables in single quotes trap "echo Took ${SECONDS}s" 0 # Prematurely expanded trap unset var[i] # Array index treated as glob 

Conditionals

ShellCheck can recognize many types of incorrect test statements.

[[ n != 0 ]] # Constant test expressions [[ -e *.mpg ]] # Existence checks of globs [[ $foo==0 ]] # Always true due to missing spaces [[ -n "$foo " ]] # Always true due to literals [[ $foo =~ "fo+" ]] # Quoted regex in =~ [ foo =~ re ] # Unsupported [ ] operators [ $1 -eq "shellcheck" ] # Numerical comparison of strings [ $n && $m ] # && in [ .. ] [ grep -q foo file ] # Command without $(..) [[ "$$file" == *.jpg ]] # Comparisons that can't succeed (( 1 -lt 2 )) # Using test operators in ((..)) [ x ] & [ y ] | [ z ] # Accidental backgrounding and piping 

Frequently misused commands

ShellCheck can recognize instances where commands are used incorrectly:

grep '*foo*' file # Globs in regex contexts find . -exec foo {} && bar {} \; # Prematurely terminated find -exec sudo echo 'Var=42' > /etc/profile # Redirecting sudo time --format=%s sleep 10 # Passing time(1) flags to time builtin while read h; do ssh "$h" uptime # Commands eating while loop input alias archive='mv $1 /backup' # Defining aliases with arguments tr -cd '[a-zA-Z0-9]' # [] around ranges in tr exec foo; echo "Done!" # Misused 'exec' find -name \*.bak -o -name \*~ -delete # Implicit precedence in find # find . -exec foo > bar \; # Redirections in find f() { whoami; }; sudo f # External use of internal functions 

Common beginner's mistakes

ShellCheck recognizes many common beginner's syntax errors:

var = 42 # Spaces around = in assignments $foo=42 # $ in assignments for $var in *; do ... # $ in for loop variables var$n="Hello" # Wrong indirect assignment echo ${var$n} # Wrong indirect reference var=(1, 2, 3) # Comma separated arrays array=( [index] = value ) # Incorrect index initialization echo $var[14] # Missing {} in array references echo "Argument 10 is $10" # Positional parameter misreference if $(myfunction); then ..; fi # Wrapping commands in $() else if othercondition; then .. # Using 'else if' f; f() { echo "hello world; } # Using function before definition [ false ] # 'false' being true if ( -f file ) # Using (..) instead of test 

Style

ShellCheck can make suggestions to improve style:

[[ -z $(find /tmp | grep mpg) ]] # Use grep -q instead a >> log; b >> log; c >> log # Use a redirection block instead echo "The time is `date`" # Use $() instead cd dir; process *; cd ..; # Use subshells instead echo $[1+2] # Use standard $((..)) instead of old $[] echo $(($RANDOM % 6)) # Don't use $ on variables in $((..)) echo "$(date)" # Useless use of echo cat file | grep foo # Useless use of cat 

Data and typing errors

ShellCheck can recognize issues related to data and typing:

args="$@" # Assigning arrays to strings files=(foo bar); echo "$files" # Referencing arrays as strings declare -A arr=(foo bar) # Associative arrays without index printf "%s\n" "Arguments: $@." # Concatenating strings and arrays [[ $# > 2 ]] # Comparing numbers as strings var=World; echo "Hello " var # Unused lowercase variables echo "Hello $name" # Unassigned lowercase variables cmd | read bar; echo $bar # Assignments in subshells cat foo | cp bar # Piping to commands that don't read printf '%s: %s\n' foo # Mismatches in printf argument count eval "${array[@]}" # Lost word boundaries in array eval for i in "${x[@]}"; do ${x[$i]} # Using array value as key 

Robustness

ShellCheck can make suggestions for improving the robustness of a script:

rm -rf "$STEAMROOT/"* # Catastrophic rm touch ./-l; ls * # Globs that could become options find . -exec sh -c 'a && b {}' \; # Find -exec shell injection printf "Hello $name" # Variables in printf format for f in $(ls *.txt); do # Iterating over ls output export MYVAR=$(cmd) # Masked exit codes case $version in 2.*) :;; 2.6.*) # Shadowed case branches 

Portability

ShellCheck will warn when using features not supported by the shebang. For example, if you set the shebang to #!/bin/sh, ShellCheck will warn about portability issues similar to checkbashisms:

echo {1..$n} # Works in ksh, but not bash/dash/sh echo {1..10} # Works in ksh and bash, but not dash/sh echo -n 42 # Works in ksh, bash and dash, undefined in sh expr match str regex # Unportable alias for `expr str : regex` trap 'exit 42' sigint # Unportable signal spec cmd &> file # Unportable redirection operator read foo < /dev/tcp/host/22 # Unportable intercepted files foo-bar() { ..; } # Undefined/unsupported function name [ $UID = 0 ] # Variable undefined in dash/sh local var=value # local is undefined in sh time sleep 1 | sleep 5 # Undefined uses of 'time' 

Miscellaneous

ShellCheck recognizes a menagerie of other issues:

PS1='\e[0;32m\$\e[0m ' # PS1 colors not in \[..\] PATH="$PATH:~/bin" # Literal tilde in $PATH rm “file” # Unicode quotes echo "Hello world" # Carriage return / DOS line endings echo hello \ # Trailing spaces after \ var=42 echo $var # Expansion of inlined environment !# bin/bash -x -e # Common shebang errors echo $((n/180*100)) # Unnecessary loss of precision ls *[:digit:].txt # Bad character class globs sed 's/foo/bar/' file > file # Redirecting to input var2=$var2 # Variable assigned to itself [ x$var = xval ] # Antiquated x-comparisons ls() { ls -l "$@"; } # Infinitely recursive wrapper alias ls='ls -l'; ls foo # Alias used before it takes effect for x; do for x; do # Nested loop uses same variable while getopts "a" f; do case $f in "b") # Unhandled getopts flags 

Testimonials

At first you're like "shellcheck is awesome" but then you're like "wtf are we still using bash"

Alexander Tarasikov, via Twitter

Ignoring issues

Issues can be ignored via environmental variable, command line, individually or globally within a file:

https://github.com/koalaman/shellcheck/wiki/Ignore

Reporting bugs

Please use the GitHub issue tracker for any bugs or feature suggestions:

https://github.com/koalaman/shellcheck/issues

Contributing

Please submit patches to code or documentation as GitHub pull requests! Check out the DevGuide on the ShellCheck Wiki.

Contributions must be licensed under the GNU GPLv3. The contributor retains the copyright.

Copyright

ShellCheck is licensed under the GNU General Public License, v3. A copy of this license is included in the file LICENSE.

Copyright 2012-2019, Vidar 'koala_man' Holen and contributors.

Happy ShellChecking!

Other Resources

• The wiki has long form descriptions for each warning, e.g. SC2221.
• ShellCheck does not attempt to enforce any kind of formatting or indenting style, so also check out shfmt!