Bash cheat sheet

bash_cheat_sheet.md

Bash Cheat Sheet

Strings

Check if a substring exists in a string

name="somethinglong"
substr="thing"
if [ ! "${name/$substr/}" = "$name" ] ; then
    echo "'$substr' found in '$name'"
else
    echo "'$substr' not found in '$name'"
fi

Arrays

Length

a=(1 2 3 4)
echo ${#a[@]}

Associative Arrays

declare -A my_dict
my_dict[foo]=bar
my_dict[some]=thing
my_dict[one]=two
for key in "${!my_dict[@]}" ; do
    printf "key $key value ${my_dict[$key]}\n"
done

Input

Read a line of input

• You almost always want to use read -r rather than just read.

  read -r COMPLEX_STRING
  printf "$COMPLEX_STRING\n"

Read line by line, in the current shell

• If you pipe a command directly to a while read ... loop, it will spawn a new subshell which cannot interact with the parent shell (for example, to change global variables).

  GLOBAL_VAL=1
  printf "line1\nline2\nline3\n" | while read -r LINE ; do
      GLOBAL_VAL=$(($GLOBAL_VAL+1))
  done
  echo $GLOBAL_VAL

  This will print 1.
• To loop in the current shell, read from the pipe using file descriptor redirection.

  GLOBAL_VAL=1
  while read -r LINE ; do
      GLOBAL_VAL=$(($GLOBAL_VAL+1))
  done < <( printf "line1\nline2\nline3\n" )
  echo $GLOBAL_VAL

  This will print 4.

Output

Printing output lines

• Using echo is usually not what you want:
  • It can fail if first argument ends up starting with "-".
  • Not all echos support the -e or -n option.
• Usually you want to use printf.
  • To print the literal string "foo\nbar":

    printf "%s" "foo\nbar"

  • To print the string and change "\n" into a newline character:

    printf "foo\nbar"