find `pwd` -name '*.js' | xargs -I jsfile acorn jsfile; echo $?

Close

Use -print0/-0 or find -exec + to allow for non-alphanumeric filenames.

Problematic code:

find . -type f | xargs md5sum

Correct code:

find . -type f -print0 | xargs -0 md5sum
find . -type f -exec md5sum {} +

Rationale:

By default, xargs interprets spaces and quotes in an unsafe and unexpected way. Whenever it's used, it should be used with -0 or --null to split on \0 bytes, and find should be made to output \0 separated filenames.

POSIX does not require find or xargs to support null terminators, so you can also use find -exec +.

Exceptions

None.

Notice

Original content from the ShellCheck https://github.com/koalaman/shellcheck/wiki.

find `pwd` -name '*.js' | xargs -I jsfile acorn jsfile; echo $?

Close

Quote this to prevent word splitting

Problematic code:

ls -l $(getfilename)

Correct code:

# getfilename outputs 1 file
ls -l "$(getfilename)"

# getfilename outputs multiple files, linefeed separated
getfilename | while IFS='' read -r line
do
  ls -l "$line"
done

Rationale:

When command expansions are unquoted, word splitting and globbing will occur. This often manifests itself by breaking when filenames contain spaces.

Trying to fix it by adding quotes or escapes to the data will not work. Instead, quote the command substitution itself.

If the command substitution outputs multiple pieces of data, use a loop instead.

Exceptions

In rare cases you actually want word splitting, such as in

gcc $(pkg-config --libs openssl) client.c

This is because pkg-config outputs -lssl -lcrypto, which you want to break up by spaces into -lssl and -lcrypto. An alternative is to put the variables to an array and expand it:

args=( $(pkg-config --libs openssl) )
gcc "${args[@]}" client.c

The power of using an array becomes evident when you want to combine, for example, the command result with user-provided arguments:

compile () {
    args=( $(pkg-config --libs openssl) "${@}" )
    gcc "${args[@]}" client.c
}
compile -DDEBUG
+ gcc -lssl -lcrypto -DDEBUG client.c

Notice

Original content from the ShellCheck https://github.com/koalaman/shellcheck/wiki.

find `pwd` -name '*.js' | xargs -I jsfile acorn jsfile; echo $?

Close

Use $(STATEMENT) instead of legacy `STATEMENT`

Problematic code

echo "Current time: `date`"

Correct code

echo "Current time: $(date)"

Rationale

Backtick command substitution `STATEMENT` is legacy syntax with several issues.

1. It has a series of undefined behaviors related to quoting in POSIX.
2. It imposes a custom escaping mode with surprising results.
3. It's exceptionally hard to nest.

$(STATEMENT) command substitution has none of these problems, and is therefore strongly encouraged.

Exceptions

None.

See also

• http://mywiki.wooledge.org/BashFAQ/082

Notice

Original content from the ShellCheck https://github.com/koalaman/shellcheck/wiki.