Bash Basic CheatSheet

Bash

Shebang

Special comment, specifies that the file is a script and calls a certain interpreter (i.e., bash, sh, python)

#!/bin/bash

#!/bin/sh

#!/usr/bin/env python

#!/usr/bin/python

Comments

Use a pound/sharp/hashtag without a ! to write comments

# This is a comment

Variables

shell variables

• Whitespace matters!
• Use $VAR to output the value of variable VAR
• Display text with echo

NAME="value"
echo "$NAME"

• Types? What types?
• Bash variables are untyped
• Operations are contextual
• Everything is string

FOO=1
$FOO + 1
# error!

• Use the expr command to evaluate expressions
• Part of coreutils

FOO=1
expr $FOO + 1
2

User input

• Use the read command get user input
• “-p” is for the optional prompt

read -p "send: " FOO
# type “hi” and enter

echo "sent: $FOO"
sent: hi

subshell

$(cmd) evaluates the command cmd inside, and substitutes the output into the script.

FOO=$(expr 1 + 1)
echo "$FOO"
2

Conditionals

test

conditional checks

• Evaluates an expression. Also synonymous with []
• Sets exit status to 0 (true), 1 (false)

-eq ==
-ne !=
-gt >
-ge >=
-lt <
-le <=

test zero = zero; echo $?
0 # 0 means true
test zero = one; echo $?
1 # 1 means false

“boolean” ops

&& and || for shell, -a and -o for test/[]

[ 0 -lt 1 ] && [ 0 -gt 1 ]; echo $?
1

[ 0 -lt 1 -o 0 -gt 1 ]; echo $?
0

if

What if…?

if [ “$1” -eq 79 ];
then
	echo “nice”
fi

if-else

…And what ifn’t

if [ “$1” -eq 79 ];
then
	echo “nice”
else
	echo “darn”
fi

elif

…And what ifn’t but if

if [ “$1” -eq 79 ];
then
	echo “nice”
elif [ “$1” -eq 42 ];
then
	echo “the answer!”
else
	echo “wat r numbers”
fi

case

No one likes long if statements…

read -p "are you 21?" ANSWER
case "$ANSWER" in
  “yes”)
    echo "i give u cookie";;
  “no”)
    echo "thats illegal";;
  “are you?”)
    echo “lets not”;;
  *)
    echo "please answer"
esac

Loops

for loops

for all your stuff in stuffs

SHEEP=("one" "dos" "tre")
for S in $SHEEP
do
	echo "$S sheep..."
done

supports ranges too

n=0
for x in {1..10}
do
	n=$(expr $x + $n)
done
echo $n

while loops

while true
do
	echo "nightmare "
done

Functions

functions

function greet() {
    echo "hey there $1"
}
greet “sysadmin decal”

hey there sysadmin decal

script args are stored the same way as with functions

# in terminal
ls .
b.txt a.txt c.txt

# script.sh
ls $1 | sort

# in terminal
./script.sh .
a.txt b.txt
c.txt

Streams

Redirection

Use > to output to somewhere else, like a text file!

echo "hello" > out.txt

Use < to take input from a file!

sort < file

Append

Use >> to append output to a file. If file is empty, works the same as >

echo "hello" >> out.txt

Pipes

Take output of first command and “pipe” it into the second one, connecting stdin and stdout

command1 | command2