Shell

• Modes
• Conditionals
• Arrays
• Loops
• Tips

Modes

mode	Descritpion
-e	Interrupts the script execution when an error occurrs (when some command returns a non-zero exit code)
-u	Immediately interrupts the script when referencing a variable that is not defined previously, with the exception of $* and $@
-o pipefail	Cause an interruption of the script when any command in a pipeline fails. By default the returning code of a command with pipeline is the retuning code of the last command used.

To set the mode just declare in the beggining of the script the command set followed by the mode:

#!/bin/bash

set -euo pipefail

...

Conditionals

Before, note that [[ ]] is a bash command that returns 0 (true) or 1 (false) (this is because of the retuning code in c main function)

Strings

Condition	Description
[[ -z STRING ]]	Empty string
[[ -n STRING ]]	Not empty string
[[ STRING == STRING ]]	Equal
[[ STRING != STRING ]]	Not equal
[[ STRING=~ STRING ]]	Regex

Numbers

Condition	Description
[[ NUM -eq NUM ]]	Equal
[[ NUM -ne NUM ]]	Not equal
[[ NUM -lt NUM ]]	Less than
[[ NUM -le NUM ]]	Less than or equal
[[ NUM -gt NUM ]]	Greater than
[[ NUM -ge NUM ]]	Greater than or equal

Files

Condition	Description
[[ -e FILE ]]	FILE exists?
[[ -r FILE ]]	Is FILE readable?
[[ -h FILE ]]	Is FILE a symlink?
[[ -d FILE ]]	Is FILE a directory?
[[ -w FILE ]]	Is FILE writable?
[[ -s FILE ]]	Is FILE not empty? (size of FILE > 0 bytes)
[[ -f FILE ]]	Is FILE a file?
[[ -x FILE ]]	Is FILE executable?
[[ FILE1 -nt FILE2 ]]	Is FILE1 more recent than FILE2?
[[ FILE -ot FILE ]]	Is FILE2 more recent than FILE1?
[[ FILE -ef FILE ]]	Same file

Arrays

Bash command	Description
$ array=(opt1 opt2 opt3)	Declaring an array (whie space separated)
${array[@]}	All elements of the array
${#array[@]}	Number of elements
${!array[@]}	Elements keys (in case of dictionaries) or indices (in case of arrays)
${array[0]}	Element 0
${array[-1]}	Last element
array=(${array[@]} opt4)	Push
array+=(opt4)	Push
unset array[1]	Unset element 1

Miscelaneous

Command	Description
allfiles=($HOME/dir/*)	Array of all file and dir names from $HOME/dir/
alllines=($(cat file))	Array of all lines from file

Loops

array=(one two three)
for element in ${array[@]}; do
	echo $element
done

for file in $HOME/mydir/*; do
	echo $file
done

Tips

• Strict mode:

  set -euo pipefail
  IFS=$'\n\t'
  

• Conditional

  if [[ -z "$STRING" ]]; then
  	echo "Empty string"
  else
  	echo "Not empty"
  fi
  

  if ping -c 1 google.com; then
  	echo "Connected"
  fi
  

• Function

  my_function(){
  	echo "Return this as a value"
  }
    
  $ my_function
  

  Alternate syntax:

  function my_function(){
  	echo "Return this as a value"
  }
    
  $ my_function
  

  Passing params to functions

  my_function(){
  	echo "Parameters passed: $1"
  }
    
  $ my_function "param"
  

• Always remember: In shell, return non-zero values considered false, zero value are considered true. Exemple:

  my_function(){
  	return 1
  }
    
  if my_function; then
  	echo "This will not be printed"
  else
  	echo "This will be printed"
  fi