2025-08-03

shell programming intro

overview of the most useful features

what shell?

the shell command language of the portable operating system interface

short: posix shell, sh

bash/ksh/zsh support posix shell but extend it with custom features.

variables

definition

a=bc

a="bc d"

no space around =

substitution

a="echo test"

$a

-> echo test

"$a"

-> "echo test"

conditionals

conditions use the exit code of commands

if grep -Fq error development.log
then
  # code
elif test $? -eq 3
  # code
else
  # code
fi

"true" is a program that is usually installed

while true
do
  echo test
done

"for" automatically splits the arguments after "in" at whitespace

for a in b c d e
do
  echo $a
done

one-liner

for a in *; do echo "$a"; done

while true; do echo test; done

special variables

last exit code

$?

command arguments

number of arguments

$#

all arguments

$@

argument 1, 2, 3, ...

$1
$2
$3

remove the first argument and move following to the left

shift

manually set the positional arguments

set -- one two three four

command lists

; ignore exit code
&& continue if exit code is 0
|| continue if exit code is not 0

example

cd /tmp && echo test || echo xy ; echo z

left associative. evaluated like this

((cd /tmp && echo test) || echo xy) ; echo z

subprocesses

& execute parralel in background
| execute command and connect output to standard input of the next command
- the two subprocesses will be connected by a pipe, which is a type of file maintained by the operating system
- the exit code of the chain is the exit code of the last command

example

ls -l | grep 'txt' | grep -v 'abc'

file output

>> create file or append to file
> create or truncate and write to file
< read from file

cat filename | tail >> output-file

sub-routines

name() {
  # code ...
}

accessing arguments

name() {
  echo $@
}

name() {
  echo $1
}

note the always empty parentheses

usage

name 3 2 1

commands

operator argument ...

file as operator. executes a file

/usr/bin/echo test

./this_file

echo

the last one searches the file in directories listed in the environment variable $PATH

variable as operator

a=echo test
$a 3

sub-routine as operator

test() {
  tail > /tmp/output
}

cat filename | test

compress_javascript() {
  uglifyjs --compress --mangle $@
}

cat input.js | compress_javascript > output.js

$@ is used in this example to be able to possibly pass additional arguments to "uglifyjs"

subshells

evaluate shell code and get what would be written on standard output as a string

$()

$(code ...)

timestamp=$(date +%s)

arithmetic

$((1 + 2))

example file

#!/bin/sh

# links config files to their destinations

install() {
  sources=$@
  cp --recursive --force --symbolic-link --verbose \
    --target-directory=/ $sources
}

if test $# -eq 0
then
  echo usage example: ./exe/install hostname
else
  install "$PWD/data/$1"/*
fi

explanations

#!/bin/sh is a so called hashbang that is used by the program execution system call to select a program to evaluate the file content with
# creates code comments that will be ignored until the end of the line
scripts should have a description at the top so that readers do not need to analyse the code to get an idea of what it is supposed to do
use long options like --option so that readers do not have to look up in man pages what single character flags like -s or combinations like -rsat stand for
the \ is used to escape a linebreak character and treat what follows on the next line as if being on the current line
for statements like "if", put "then" on the following line, to not have to add an extra semicolon like "if; then"
"test" is a program that is usually installed that is also aliased as [

to execute a file

./filename
sh filename