Shell: Cheat sheet

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Basics

Variable

Display: echo "$var"

Decision making

• If
  • C/JS like: if (( $var == 0 )); then...; fi
  • Normal: if [[ $var -eq $var2 ]]; then...; fi
  • Is it in?:
    • if [[ $VAR == *"Linux"* ]]
    • if [[ $VAR =~ .*Linux.* ]]
• Switch:
  case ... in
...) ... ;;
esac
• Is a string empty?: -z
• Everything else is the same as other language using (( ... ))

Loop

• For:
  • for; do...; done
  • C/JS like: for (( i = 0; i < 10; i++))
  • Python like: for i in {0..2}
• While: running as long as condition is true
  • while ... option; do ...; done
• Until: running as long as condition is false
  • until ...; do ...; done

Array

• Declare it: a=() or initialized it a=(1 2 5 6 8 67)
• Get value at an index: ${a[0]}
• Get length: ${#a[@]}
• Add/get a value in following index in a loop: tmp[${#tmp[@]}]=
• Get all values: ${z[*]} or ${z[@]} (can be used in loop to retrieve values not index)
• Copy of array: z=("${tmp[@]}")

Operation

• Calculate: $((i + 1))

Functions

• Declare a function: function function_A {...}
• Get parameters & call a function is the same is calling a script and passing args
  • Get parameters: 1rst param $1, 2nd param $2, ...
  • call function: function_A arg1 arg2

String

see basics/string_basic_operation.sh

Advanced

Special variables

• $0 : filename
• $n: Nth arg passed was invoked or function was called
  test it: if [ "$1" != "" ]
• $#: nbr of arg passed (script or function)
  there are arguments if > 0
• $@ or $*: all args passed (script or function)
  can loop on it
• $?: exit status from last command executed
• $$: process ID of shell (shell scripts: process ID under which they are executing)
• $!: process nbr of last bg command

Trap command

Catch special signal/interruption/user input in script to prevent the unpredictables trap <arg/function> <signal>
ex: trap "echo Booh!" SIGINT SIGTERM --> on Crtl+C displays Booh!

File testing

• File exists?: if [ -e "$filename" ]
• Directory exists?: if [ -d "$directory_name" ]
• Read permission?: if [ ! -f "$filename" ]

Input Parameter Parsing

• To get arguments see Special Variables section
• Read option values: loop on getops function
  • ex: get value for -u -a -l
while getopts u:a:l: option
do
case "${option}" in
u) USER=${OPTARG};;
a) AGE=${OPTARG};;
l) LOCATION=${OPTARG};;
esac
done

Pipe(line)s

• Counts nbr entries in directory: ls / | wc -l
• Get 1rst 10 results: ls / | head outputs the first 10 lines by default, use option -n to change
  Remark: By default pipelines redirects only the standard output, if you want to include the standard error you need to use the form |& which is a short hand for 2>&1 |

Redirection ~= Process Substitution

• Save sorted results to new files & show diffs between files:
  • without process subs: sort file1 > sorted_file1
sort file2 > sorted_file2
diff sorted_file1 sorted_file2
  • with: diff <(sort file1) <(sort file2)
• Store logs app into file & print it in console: echo "Hello, world!" | tee /tmp/hello.txt
• Only lower case in file but keep regular case on output: echo "Hello, world!" | tee >(tr '[:upper:]' '[:lower:]' > /tmp/hello.txt)