Hidden features of Bash

insert preceding line's final parameter

alt-. the most useful key combination ever, try it and see, for some reason no one knows about this one.

press it again and again to select older last parameters.

great when you want to do something else to something you used just a moment ago.

If you want to keep a process running after you log out:

disown -h <pid>

is a useful bash built-in. Unlike nohup, you can run disown on an already-running process.

First, stop your job with control-Z, get the pid from ps (or use echo $!), use bg to send it to the background, then use disown with the -h flag.

Don't forget to background your job or it will be killed when you logout.

Almost everything listed under EXPANSION section in the manual

In particular, parameter expansion:

$ I=foobar
$ echo ${I/oo/aa} #replacement
faabar
$ echo ${I:1:2}   #substring
oo
$ echo ${I%bar}   #trailing substitution
foo
$ echo ${I#foo}   #leading substitution
bar

My favorite:

sudo !!

Rerun the previous command with sudo.

Get back history commands and arguments

It's possible to selectively access previous commands and arguments using the ! operator. It's very useful when you are working with long paths.

You can check your last commands with history.

You can use previous commands with !<n> being n the index of the command in history, negative numbers count backwards from the last command in history.

ls -l foo bar
touch foo bar
!-2

You can use previous arguments with !:<n>, zero is the command, >= 1 are the arguments.

ls -l foo
touch !:2
cp !:1 bar

And you can combine both with !<n>:<m>

touch foo bar
ls -l !:1 !:2
rm !-2:1 !-2:2
!-2

You can also use argument ranges !<n>:<x>-<y>

touch boo far
ls -l !:1-2

Other ! special modifiers are:

• * for all the arguments

  ls -l foo bar
  ls !*
  

• ^ for the first argument (!:1 == !^)

• $ for the last argument

  ls -l foo bar
  cat !$ > /dev/null
  

I like the -x feature, allowing to see what's going on in your script.

bash -x script.sh 

SECONDS=0; sleep 5 ; echo "that took approximately $SECONDS seconds"

SECONDS

Each time this parameter is referenced, the number of seconds since shell invocation is returned. If a value is assigned to SECONDS, the value returned upon subsequent references is the number of seconds since the assignment plus the value assigned. If SECONDS is unset, it loses its special properties, even if it is subsequently reset.

Here is one of my favorites. This sets tab completion to not be case sensitive. It's really great for quickly typing directory paths, especially on a Mac where the file system is not case sensitive by default. I put this in .inputrc in my home folder.

set completion-ignore-case on

The special variable random:

if [[ $(($RANDOM % 6)) = 0 ]]
    then echo "BANG"
else
    echo "Try again"
fi   

Regular expression handling

Recent bash releases feature regular expression matching, so you can do:

if [[ "mystring" =~ REGEX ]] ; then  
    echo match
fi

where REGEX is a raw regular expression in the format described by man re_format.

Matches from any bracketed parts are stored in the BASH_REMATCH array, starting at element 1 (element 0 is the matched string in its entirety), so you can use this to do regex-powered parsing too.

Quick & Dirty correction of typos (especially useful for long commands over slow connections where using the command history and scrolling through it would be horrible):

$ cat /proc/cupinfo
cat: /proc/cupinfo: No such file or directory
$ ^cup^cpu

Also try !:s/old/new which substitutes old with new in the previous command once.

If you want to substitute many occurrences you can do a global substitution with !:gs/old/new.

You can use the gs and s commands with any history event, e.g.

!-2:s/old/new

To substitute old with new (once) in the second to last command.

Ctrlx Ctrle

This will load the current command into the editor defined in the variable VISUAL. This is really useful for long commands like some of those listed here.

To use vi as your editor:

export VISUAL=vi

Here two of my favorites:

To check the syntax w/o really executing the script use:

bash -n script.sh

Go back to the last directory (yes I know pushd and popd, but this is quicker)

cd -

Using Infix Boolean Operators

Consider the simple if:

if [ 2 -lt 3 ]
    then echo "Numbers are still good!"
fi

That -lt looks kinda ugly. Not very modern. If you use double brackets around your boolean expression you can the normal boolean operators!

if [[ 2 < 3 ]]
    then echo "Numbers are still good!"
fi

Arrays:

#!/bin/bash

array[0]="a string"
array[1]="a string with spaces and \"quotation\" marks in it"
array[2]="a string with spaces, \"quotation marks\" and (parenthesis) in it"

echo "There are ${#array[*]} elements in the array."
for n in "${array[@]}"; do
    echo "element = >>${n}<<"
done

More details on arrays (and other advanced bash scripting stuff) can be found in the Advanced Bash-Scripting Guide.

Running a command before displaying the bash prompt

Set a command in the "PROMPT_COMMAND" env variable and it will be run automatically before each prompt. Example:

[lsc@home]$ export PROMPT_COMMAND="date"
Fri Jun  5 15:19:18 BST 2009
[lsc@home]$ ls
file_a  file_b  file_c
Fri Jun  5 15:19:19 BST 2009
[lsc@home]$ ls

For the next april fools, add "export PROMPT_COMMAND=cd" to someone's .bashrc then sit back and watch the confusion unfold.

export TMOUT=$((15*60))

Terminate bash after 15 minutes of idle time, set to 0 to disable. I usually put this to ~/.bashrc on my root accounts. It's handy when administrating your boxes and you may forget to logout before walking away from the terminal.

Using arithmetic:

if [[ $((2+1)) = $((1+2)) ]]
    then echo "still ok"
fi

I recently read Csh Programming Considered Harmful which contained this astounding gem:

Consider the pipeline:

A | B | C

You want to know the status of C, well, that's easy: it's in $?, or $status in csh. But if you want it from A, you're out of luck -- if you're in the csh, that is. In the Bourne shell, you can get it, although doing so is a bit tricky. Here's something I had to do where I ran dd's stderr into a grep -v pipe to get rid of the records in/out noise, but had to return the dd's exit status, not the grep's:

device=/dev/rmt8
dd_noise='^[0-9]+\+[0-9]+ records (in|out)$'
exec 3>&1
status=`((dd if=$device ibs=64k 2>&1 1>&3 3>&- 4>&-; echo $? >&4) |
    egrep -v "$dd_noise" 1>&2 3>&- 4>&-) 4>&1`
exit $status;

Truncate content of a file (zeroing file)

> file

Specifically, this is very good for truncating log files, when the file is open by another process, which still may write to the file.

Not really a feature but rather a direction: I found many "hidden features", secrets and various bash usefulness at commandlinefu.com. Many of the highest rated answers to this answers, I learned them on that site :)

C style numeric expressions:

let x="RANDOM%2**8"
echo -n "$x = 0b"
for ((i=8; i>=0; i--)); do
  let n="2**i"
  if (( (x&n) == n )); then echo -n "1"
  else echo -n "0"
  fi
done
echo ""

These properties are another one of my favorites.

export HISTCONTROL=erasedups
export HISTSIZE=1000

The first one makes sure bash doesn't log commands more than once, will really improves history's usefulness. The other expands the history size to 1000 from the default of 100. I actually set this to 10000 on my machines.

Easily move around between multiple directories

Not a hidden feature, but much more flexible than pushd which requires stack-like navigation.

a() { alias $1=cd\ $PWD; }

cd somewhere and type a 1. Later on just typing 1 will return to that directory.

Using 'let' built-in bash command for basic arithmetic

A=10
let B="A * 10 + 1" # B=101
let B="B / 8"      # B=12, let does not do floating point
let B="(RANDOM % 6) + 1" # B is now a random number between 1 and 6

To do floating point evaluations, you can use the "bc" command (no part of bash).

FP=`echo "scale=4; 10 / 3" | bc` # FP="3.3333"

Embedded Command substitution:

hostname && dig +short $(hostname) && dig +short -x $(dig +short $(hostname))

This command is good for checking RDNS on your mail server. :P