Shell scripts are often used as glue, for automation and simple one-off tasks. What are some of your favorite "hidden" features of the Bash shell/scripting language?
See also:
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
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;
The special variable random:
if [[ $(($RANDOM % 6)) = 0 ]]
then echo "BANG"
else
echo "Try again"
fi
I like the -x feature, allowing to see what's going on in your script.
bash -x script.sh
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
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.
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 -
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>
ls -l foo bar
touch !:2 !:3
rm !-2:2 !-2:3
!-3
Another ! 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
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 ""
Not that is is a hidden feature. I think it dosn't exist, but it would be magic to have a special syntax allowing to work on thinks on distinct machine. Something like
cat file1 > machine2:file1
that would copy file1 to you directory on machine2. You can also imagine running programs on distinct machines. It would be something like a hidden ssh connection.
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.
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.
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
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.
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.
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
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.
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.
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"
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.
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
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\ $PWD; }
cd somewhere and type "a 1". Later on just typing "1" will return to that directory.
One I use a lot is !$ to refer to the last word of the last command:
$ less foobar.txt
...
# I dont want that file any more
$ rm !$
As others have mentioned, Ctrl-r is great for stepping back through your command history. But what if you want to go forward after you've taken one or a few steps too many? That's where Ctrl-s comes in handy. However, it's normally mapped to XOFF (interrupt data flow). Since that's not too useful any more because we're not using slow serial terminals, you can turn off that mapping with:
stty -ixon
in your ~/.bashrc file.
This also makes Ctrl-q available which is normally a duplicate of Ctrl-v (quoted-insert which allows you to insert a literal control character). I have Ctrl-q mapped to menu-complete which steps through completions when pressed repeatedly. I like to leave Tab set to regular complete.
You can set Ctrl-q to menu-complete by adding this line to your ~/.inputrc file:
"\C-q": menu-complete
Bash has variable indirection:
$ foo=bar
$ baz=foo
$ echo ${!baz}
bar
I have an alias r='fc-s', and I find it very useful in some limited cases. To run the last command, just type r and hit enter, and that's it. Of course, that itself is not very useful because up arrow does the same thing. But you can use r to run the previous command with substitutions. Let's say your last command was a long command compiling some file:
$ gcc -c <file_name>.c <lots of options> -o <file_name>.o
Now you want to compile another file with the same options and have a corresponding .o file:
$ r <file_name>=<new_file>
will do it. You don't have to use up arrow, navigate to the right places and then replace them each manually. This can be repeated multiple times, so you can do this next:
$ r <new_file>=<other_file>
Of course, for such a thing you have makefiles, but I hope I have shown that the alias is useful.
I haven't needed the use of this alias a lot, but there have been times that I have been glad that I have this alias!
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.
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 :)
set -o vi in order to have vi-like editing of the command history as well as of the currently typed command.
C-S-- Control Shift Minus Undo-es typing actions.
Any delete operation C-w (delete previous word), C-k (delete to end of line), C-u (delete to start of line) etc... copies it's deleted text to the kill ring, you can paste the last kill with: C-y and cycle through (and paste from) the ring of deleted items with Alt-y
You can ignore certain files while tab completing by setting th FIGNORE variable.
For example, if you have a subverion repo and you want to navigate more easily do
export FIGNORE=".svn"
now you can cd without being blocked by .svn directories.