Shell Cheatsheet

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Assigning Variables

#!/bin/sh
length=80
chapter=chap # no spaces
title="Greetings from Space" # with spaces
empty= # empty variable
size=$length

Displaying Variables

#!/bin/sh
echo $length # 80
echo length=$length # length=80
echo this is $title # this is Greetings from Space
echo ${chapter}5 # chap5 - to display with no space between variable and other parts of string

Special Variables

#!/bin/sh
$? # exit status of the last command
$$ # process number of the current shell
$! # process number of the last background command
$0 # filename of the current script
$# # number of arguments
$n # arguments (n = 1, 2, ...)
$@ # equivalent of $1 $2 ... $n
$* # same as $@

Special Symbols and Escaping

  • Double quotes escape special characters, except for $, `, \c (c = any character)
  • Single quotes escapes $ as well
  • Backslash escapes single character
  • Combinations with backsllash:
    • \n – new line
    • \c – stay on the same line
    • \t – tab

#!/bin/sh
echo * # equivalent to ls (print out current dir)
echo "*" # prints *
echo "$length" # prints 80 - $ is not escaped by double quotes
echo '$length' # prints $length
echo \$length # prints $length - escapes single character

Reverse quotes (`command`) – run command:

#!/bin/sh
echo today is `date` # shows today's date
users=`who | wc -l`
echo $users # displays number of users logged in

Arguments Shift

#!/bin/sh
# script called like this:
# thisscript hello world
a=$1; shift
b=$1
echo $b $a # world hello

Operators

&& - second command will only run if the first one was successful
|| - second command runs only if first one fails

#!/bin/sh
test -d ~/b && echo "b is directory" # will show "b is directory" only if b is directory
test -f ~/b || echo "b is not file, therefore b is directory" # equivalent to above

For and list

#!/bin/sh
for i in here there everywhere # list
do
   echo $i
done
# will print 
# > here
# > there
# > everywhere

Lists are separated by ; or new line

If-else

#!/bin/sh
if [condition1]
then
    echo "condition 1"
else
    echo "condition 2"
fi

# or
if [condition1]; then
    echo "condition 1"
else
    echo "condition 2"
fi
Conditions
  • -a - and
  • -o - or
  • -gt - greater then
  • -lt - less then
  • -eq - equal
  • -ne - not equal
  • -ge - greater or equal
  • -le - less or equal

#!/bin/sh
if [ $i -gt 7 -a $i -le 99] #$i between 7 and 99

While and Until

#!/bin/sh
# While
while [condition]; do
  [command]
done

# Until
until [condition]; do
  [command]
done

Case

#!/bin/sh
case $i
in
  1) continue;;
  2) echo $i
     continue;;
  3) break;;
esac

Arithmetic Operations (expr)

#!/bin/sh
n=0
n=$n+1
echo $n # prints 0+1 - treats as string by default

n=0
n=`expr $n+1`
echo $n # prints 1

n=`expr 2+3`
echo $n # prints 5
expr 5 "*" 6 # 30
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