Bash

Warning

This sheet is a supplement for my reference, so might not be useful to others

File Downloads¶

wget¶

Wget to location

wget -O savefile.txt http://web/file.txt

Comparing files¶

comm - column display of file diferences
diff - You know, the most common diff format

Diff¶

Flag	Description
-y	Side by side
-r	Recursive (directory)

Scripting¶

Special Variables¶

Var	Description
`$0`	Name of bash script
`$1` - `$9`	First 9 arguments of script
`$#`	Number of arguments
`$?`	Status code of last run process
`$$`	Process ID of current script
`$USER`	Current user
`$HOSTNAME`	Hostname of machine
`$RANDOM`	Random integer
`$LINENO`	Current line number in script

Reading Input¶

echo "What is your name?"
read name
echo "Hello, $name"

Flag	Description
-p "Prompt"	Specify prompt
-s	Don't echo user input, eg. for passwords

Conditionals¶

If Statement

if [ <some test> ]; then
  do something
fi

Else Statement

if [ <some test> ]; then
  do something
else
  do somethingelse
fi

Warning

Conditionals are very white space sensitive

Operator	Description (True if)
`!EXPRESSION`	The EXPRESSION is false
`-n STRING`	STRING length > 0
`-z STRING`	STRING length = 0
`STRING1 != STRING2`	Strings not equal
`STRING1 = STRING2`	Strings equal
`INT1 -eq INT2`	Int equality
`INT1 -ne INT2`	Int inequality
`INT1 -gt INT2`	Int1 > INT2
`INT1 -lt INT2`	Int1 < INT2
`INT1 -ge INT2`	Int1 >= INT2
`INT1 -le INT2`	Int1 <= INT2
`-d PATH`	Path exists and is a directory
`-e PATH`	Path exists
`-r PATH`	Path exists and is readable
`-w PATH`	Path exists and is writable
`-x PATH`	Path exists and is executable

[ is actually a program, which returns 1 for a false, or 0 for true.

As such you can do

if some command; then
  echo "Command succeeded"
else
  echo "Command failed"
fi

Boolean Operators¶

command1 && command2

Will run command2 if and only if command1 succeeds (returns zero code)

command1 || command2

Will run command2 if and only if command1 fails (returns non-zero code)

Example

grep $user2 /etc/passwd && echo "$user2 found!" || echo "$user2 not found!"

Loops¶

For Loops¶

Syntax

for var in <list>
do
  echo $var
done

Example

for ip in $(seq 1 10); do echo 10.11.1.$ip; done

or

for ip in {1..10}; do echo 10.11.1.$ip; done

Example 2

for textfile in *.txt; do echo "Found $textfile"; done

While loops¶

Syntax

while [ <some test> ]
do
  something
done

Previous for loop example

counter=1
while [ $counter -lt 10 ]
do
  echo "10.11.1.$counter"
  ((counter++))
done

Functions¶

Syntax

function my_func {

}

Variables¶

Two types of variables, shell variables and environment variables.

Shell variables are not passed to child processes

shell_var="stuff"
echo "shell_var=${shell_var}"

Environment variables are inherited by children and can be seen in /proc/<pid>/environ

Can set an environment variable with export

export SOME_ENV="stuff"
env | grep "SOME_ENV" # returns 'SOME_ENV=stuff'
ANOTHER_ENV="something else"
env | grep "ANOTHER_ENV" # Doesn't work
export ANOTHER_ENV
env | grep "ANOTHER_ENV" # returns 'ANOTHER_ENV=something else'

Useful Commands¶

xdg-open¶

Open the file or folder in whatever program is the defaul, eg. open a media player for a video, or your browser for a website or a file browser for a directory.

Last update: 2021-09-01