How To Check Syntax Errors In Shell Script

How to check error in Shell Script. Command to check syntax error in Shell Script. How to debug a Shell Script in Linux Ubuntu Systems. Install ShellCheck on Ubuntu. ShellCheck is a static analysis tool for shell scripts. It is an easy way to check whether the Shell Script contains syntax errors. 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.

What ShellCheck Can Detect

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

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 ((..))

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
f() { whoami; }; sudo f # External use of internal functions

Common 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 “Argument 10 is $10” # Positional parameter misreference
if $(myfunction); then ..; fi # Wrapping commands in $()
else if othercondition; then .. # Using ‘else if’

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 “%sn” “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

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

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
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

Note: ShellCheck makes canonical use of exit codes, and can output simple JSON, CheckStyle compatible XML, GCC compatible warnings as well as human readable text (with or without ANSI colors).

Install ShellCheck

Run the following commands to install ShellCheck on Ubuntu and Debian Systems:

apt-get update
apt-get install shellcheck

How To Use ShellCheck

On the web

  • Paste a shell script on http://www.shellcheck.net for instant feedback.
  • ShellCheck.net is always synchronized to the latest git commit, and is the simplest 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 Syntastic.
  • Atom, through Linter.
  • Sublime, through SublimeLinter.
  • Emacs, through Flycheck.
  • Most other editors, through GCC error compatibility.

How To Check Syntax Errors In Shell Script originally posted on Source Digit - Latest Technology, Gadgets & Gizmos.

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