A bunch of things about shell scripting i always forget. Writing it down helps me to keep that in mind.

SH vs Bash

Bash (Bourne-again shell) is basically a superset of SH (Bourne shell) and has more functionality (e.g. arrays). On a few systems the sh command is just symlink to bash. But bash behaves more POSIX compliant when it’s called via sh. Look also at this Stackoverflow post.

SH scripting

The sh scripting language is very simple. Almost everything in sh is a command or a symlink to a command. There are variables, conditionals, loops and functions. You can type commands directly in your terminal or put it in a file. The script can then be executed with sh file.sh.

You can also put a shebang on the first line #!/bin/sh and give execution rights to the file chmod +x file.sh. Now you can execute your script with ./file.sh.


In sh you can define variables in the scope of a script like this: FOO=1. If you want them persist over a terminal session you can type export FOO=1. To return their values type echo $FOO.


${BAR:-bar} # default value "bar" for BAR, can be set from outside the script

echo $FOO $BAR # prints: foo bar
echo ${FOO}baz # prints foobaz
echo ${FOO} # prints string length of "foo"

${BAZ:?variable BAZ missing} # exits script if variable BAZ is missing 

echo $BAZ

To execute the script above prepend a BAR=blabla in front of script.sh or the script exits with message “variable BAR missing”.

Type env or export -p into your terminal to see all environment variables.


Commandline arguments you can use inside a script. Example shows the execution of a script foo.sh with arguments one two. The output will remain the same dispite executing with sh foo.sh one two or ./foo.sh one two except $0.

echo Scriptname: $0
echo Number of args: $#
echo All args: $@
echo All args expanded: "$@"
echo First arg: $1
echo Second arg: $2
echo Process number: $$
echo Last command exit code: $?
sh -stfu 2> /dev/null # silent stderr
echo Last command exit code: $?


Scriptname: ./foo.sh
Number of args: 2
All args: one two
All args expanded: one two
First arg: one
Second arg: two
Process number: 15442
Last command exit code: 0
Last command exit code: 2

Conditional - the test command

The sh if takes a command as parameter, e.g. if ls; then ...; fi. If the command exits with 0 the condition is true. To check a condition we have a progam called test. test 2 -lt 3 would exit with code 0 because 2 is lower then 3. With this program we can check conditions in a if statement:

if test 2 -lt 3
  then echo "2<3"

There’s also the shorthand [ for test. [ is a simple symlink to test. When test gets called via [ it expects a closing ]. Now we can write scripts like:

if [ 2 -lt 3 ]
  then echo "2<3"

Following example shows a few capabilities of the test command. For more see the wikipedia article. Execute the script with ./foo.sh 5:


# if/else
if [ $1 -le 2 ] 
  then echo "$1 <= 2"
elif [ $1 -le 4 ]
  then echo "$1 <= 4"
  echo "i can't count to $1"

# numbers
if [ 1 -eq "1" ]; then echo "1 == 1"; fi  # equal
if [ 1 -ne 2 ]; then echo "1 != 2"; fi    # not equal
if [ 2 -gt 1 ]; then echo "2 > 1"; fi     # greater then
if [ 2 -ge 2 ]; then echo "2 >= 2"; fi    # greater equal
if [ 1 -lt 2 ]; then echo "1 < 2"; fi     # lower then
if [ 1 -le 1 ]; then echo "1 <= 2"; fi    # lower equal

# strings
if [ "foo" = "foo" ]; then echo '"foo" == "foo"'; fi
if [ "foo" != "bar" ]; then echo '"foo" != "bar"'; fi
if [ -z "" ]; then echo '"" is empty'; fi
if [ -n "foo" ]; then echo '"foo" is not empty'; fi

# always use "$VAR" in quotes if you checking string variables
if [ "$1" = "5" ]; then echo "$1 == 5"; else echo "$1 != 5"; fi

# check folders and files
if [ -f "$0" ]; then echo "me exists"; fi
mkdir foo
if [ -d "./foo" ]; then echo "folder exists"; fi
rm -r foo
if [ ! -d "./foo" ]; then echo "folder removed"; fi


Expected! There are loops in sh. Nice, let’s see how they work.


# while loop with break condition
while [ $i -le 10 ]
  echo $i
  i=$(( $i + 1 ))

# infinite loop 
while [ : ]
  echo 1
  break # we don't want you to run infinite :)

# for loop with list [one, two, three]
for i in one two three
  echo $i

# for loop with list from command (all files/dirs in $PWD that end with .sh)
for i in $(ls *.sh)
  echo $i

# loop through all file names in $PWD that end with .sh
for i in *.sh
 echo $i

# loop through all command line arguments
for i in $@
  echo $i

# same as above, but shorter
for i
 echo $i 


To avoid writing repetitive tasks there are functions in sh. Function arguments are not taken via foo(1, "two"), instead functions work like normal sh commands, parameters are passed via $@, $1, $2, etc.


hello() {
  echo "hello $@"

hello world !