my bash cheat sheet

bash.md

Bash cheat sheet

Only the most common stuff

• Index:
  • Script dir
  • Required / default vars
  • Pass all arguments
  • Arrays
  • Strings
• Ref:
  • Gnu bash manual
    • Conditional Expressions
  • Posix (Open Group / SUS)

Script dir

# Doesn't work if "this file" is a symlink
SCRIPT_DIR=$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" &> /dev/null && pwd )

# Doesn't work if you're trying to "source" this script
SCRIPT_DIR=$(dirname -- "$(readlink -f -- "$0";)";)

# Works everywhere, but since this does `cd` out you may not be able to `cd` back in
pushd . > '/dev/null';
SCRIPT_PATH="${BASH_SOURCE[0]:-$0}";
while [ -h "$SCRIPT_PATH" ];
do
    cd "$(dirname -- "$SCRIPT_PATH")";
    SCRIPT_PATH="$(readlink -f -- "$SCRIPT_PATH")";
done
cd "$(dirname -- "$SCRIPT_PATH")" > '/dev/null';
SCRIPT_PATH="$(pwd)";
popd > '/dev/null';

Required / default vars

• Parameter Expansion

: "${REQVAR:?You must set REQVAR}"
: "${MAYBEVAR:='default value'}"

Pass all arguments

• Special Parameters

#!/bin/bash
otherprog "$@"   # equvalent to "$1" "$2" ...
otherprog $*     # same thing
otherprog "$*"   # equivalent to "$1 $2" 

All but the first:

#!/bin/bash
shift
otherprog "$@"   # equvalent to "$2" "$3" ...

Arrays

Declare

my_arr=(foo bar baz)
my_arr+=(bing boom)    #append
arr_size=${#my_arr[@]} #count

Iterate

# By value
for v in "${my_arr[@]}"; do
    echo $v
done

# By index
for i in "${!my_arr[@]}"; do
  echo ${my_arr[$i]}
done

Delete matching (only first match)

# delete the 'foo' element from my_arr
my_arr=( "${my_arr[@]/foo}" )

or

delete='foo'
my_arr=( "${my_arr[@]/$delete}" )

Pass to function

You can't. You have two options:

1. Treat it like a vararg, passing the expanded version:

   function foo () {
       local bar="$1"
       shift
       local baz="$1"
       shift
       local rest=("$@")
   }
   
   neet=(bing bang boom)
   foo "bar" "baz" "${neet[@]}"

2. Pass it as a nameref:

   function foo () {
       local bar="$1"
       local baz="$2"
       local -n rest="$3"
   }
   
   neet=(bing bang boom)
   foo "bar" "baz" "neet"

Strings

trim whitespace

FOO=$(echo "$FOO" | xargs)