Questions tagged [bash]

Questions specific to GNU’s Bourne Again SHell, as opposed to other Bourne/POSIX shells. For questions about Unix shells in general, use the /shell tag instead.

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909
votes
11answers
2.2m views

How to correctly add a path to PATH?

I'm wondering where a new path has to be added to the PATH environment variable. I know this can be accomplished by editing .bashrc (for example), but it's not clear how to do this. This way: export ...
522
votes
20answers
176k views

Preserve bash history in multiple terminal windows

I consistently have more than one terminal open. Anywhere from two to ten, doing various bits and bobs. Now let's say I restart and open up another set of terminals. Some remember certain things, some ...
437
votes
4answers
288k views

How to cycle through reverse-i-search in BASH?

In the terminal, I can type CTRL-R to search for a matching command previously typed in BASH. E.g., if I type CTRL-R then grep is lists my last grep command, and I can hit enter to use it. This only ...
418
votes
3answers
300k views

Using “${a:-b}” for variable assignment in scripts

I have been looking at a few scripts other people wrote (specifically Red Hat), and a lot of their variables are assigned using the following notation VARIABLE1="${VARIABLE1:-some_val}" or some expand ...
407
votes
25answers
560k views

How can I resolve a hostname to an IP address in a Bash script?

What's the most concise way to resolve a hostname to an IP address in a Bash script? I'm using Arch Linux.
354
votes
14answers
58k views

In Bash, when to alias, when to script, and when to write a function?

It's taken me almost 10 years of Linux usage to ask this question. It was all trial and error and random late-night internet surfing. But people shouldn't need 10 years for this. If I were just ...
336
votes
14answers
448k views

How to get execution time of a script effectively?

I would like to display the completion time of a script. What I currently do is - #!/bin/bash date ## echo the date at start # the script contents date ## echo the date at end This just show's ...
281
votes
4answers
219k views

Why does my shell script choke on whitespace or other special characters?

Or, an introductory guide to robust filename handling and other string passing in shell scripts. I wrote a shell script which works well most of the time. But it chokes on some inputs (e.g. on some ...
276
votes
10answers
479k views

How to conditionally do something if a command succeeded or failed

How can I do something like this in bash? if "`command` returns any error"; then echo "Returned an error" else echo "Proceed..." fi
269
votes
4answers
61k views

What does “rc” in .bashrc stand for?

Is it "resource configuration", by any chance?
259
votes
21answers
664k views

Colorizing your terminal and shell environment?

I spend most of my time working in Unix environments and using terminal emulators. I try to use color on the command line, because color makes the output more useful and intuitive. What options exist ...
254
votes
9answers
153k views

How to colorize output of git?

Is there a way to color output for git (or any command)? Consider: baller@Laptop:~/rails/spunky-monkey$ git status # On branch new-message-types # Changes not staged for commit: # (use "git add &...
254
votes
4answers
68k views

How do I clear Bash's cache of paths to executables?

When I execute a program without specifying the full path to the executable, and Bash must search the directories in $PATH to find the binary, it seems that Bash remembers the path in some sort of ...
249
votes
4answers
297k views

How to get the pid of the last executed command in shell script?

I want to have a shell script like this: my-app & echo $my-app-pid But I do not know how the get the pid of the just executed command. I know I can just use the jobs -p my-app command to grep ...
245
votes
12answers
380k views

How do I loop through only directories in bash?

I have a folder with some directories and some files (some are hidden, beginning with dot). for d in *; do echo $d done will loop through all files, but I want to loop only through directories. How ...
240
votes
8answers
468k views

How can I test if a variable is empty or contains only spaces?

The following bash syntax verifies if param isn't empty: [[ ! -z $param ]] For example: param="" [[ ! -z $param ]] && echo "I am not zero" No output and its fine. But when param is ...
239
votes
6answers
87k views

What is the difference between the Bash operators [[ vs [ vs ( vs ((?

I am a little bit confused on what do these operators do differently when used in bash (brackets, double brackets, parenthesis and double parenthesis). [[ , [ , ( , (( I have seen people use them on ...
236
votes
14answers
420k views

How can I get the size of a file in a bash script?

How can I get the size of a file in a bash script? How do I assign this to a bash variable so I can use it later?
236
votes
5answers
109k views

What does env x='() { :;}; command' bash do and why is it insecure?

There is apparently a vulnerability (CVE-2014-6271) in bash: Bash specially crafted environment variables code injection attack I am trying to figure out what is happening, but I'm not entirely sure ...
212
votes
4answers
290k views

How to define 'tab' delimiter with 'cut' in BASH?

Here is an example of using cut to break input into fields using a space delimiter, and obtaining the second field: cut -f2 -d' ' How can the delimiter be defined as a tab, instead of a space?
211
votes
5answers
162k views

Why doesn't my Bash script recognize aliases?

In my ~/.bashrc file reside two definitions: commandA, which is an alias to a longer path commandB, which is an alias to a Bash script I want to process the same file with these two commands, so I ...
206
votes
3answers
18k views

Security implications of forgetting to quote a variable in bash/POSIX shells

If you've been following unix.stackexchange.com for a while, you should hopefully know by now that leaving a variable unquoted in list context (as in echo $var) in Bourne/POSIX shells (zsh being the ...
201
votes
7answers
205k views

How to remove a single line from history?

I'm working in Mac OSX, so I guess I'm using bash...? Sometimes I enter something that I don't want to be remembered in the history. How do I remove it?
197
votes
9answers
293k views

How to view the output of a running process in another bash session?

I have left a script running on a remote machine from when I was locally working at it. I can connect over SSH to the machine as the same user and see the script running in ps. $ ps aux | grep ...
189
votes
20answers
263k views

How to add a newline to the end of a file?

Using version control systems I get annoyed at the noise when the diff says No newline at end of file. So I was wondering: How to add a newline at the end of a file to get rid of those messages?
174
votes
7answers
475k views

What is the “eval” command in bash?

What can you do with the eval command? Why is it useful? Is it some kind of a built-in function in bash? There is no man page for it..
170
votes
5answers
571k views

How can I get the current working directory? [duplicate]

I want to have a script that takes the current working directory to a variable. The section that needs the directory is like this dir = pwd. It just prints pwd how do I get the current working ...
170
votes
3answers
154k views

Bash - How can I make “Press any key to continue” [duplicate]

I'm making a script to install my theme, after it finished installing it will appear the changelog and there will be "Press any key to continue" so that after users read the changelog then press any ...
169
votes
7answers
90k views

Terminal prompt not wrapping correctly

I have an issue where if I type in very long commands in bash the terminal will not render what I'm typing correctly. I'd expect that if I had a command like the following: username@someserver ~/...
164
votes
12answers
272k views

How do I change the extension of multiple files?

I would like to change a file extension from *.txt to *.text. I tried using the basename command, but I'm having trouble on changing more than one file. Here's my code: files=`ls -1 *.txt` for x in ...
162
votes
6answers
34k views

Run a command that is shadowed by an alias

Let's say I have the following alias in bash - alias ls='ls --color=auto' - and I want to call ordinary ls without options. Is the only way to do that is to unalias, do the command and then alias ...
159
votes
10answers
187k views

Execute a command once per line of piped input?

I want to run a java command once for every match of ls | grep pattern -. In this case, I think I could do find pattern -exec java MyProg '{}' \; but I'm curious about the general case - is there an ...
159
votes
4answers
200k views

What is the meaning of IFS=$'\n' in bash scripting?

At the beginning of a bash shell script is the following line: IFS=$'\n' What is the meaning behind this collection of symbols?
155
votes
2answers
244k views

There are stopped jobs (on bash exit)

I get the message There are stopped jobs. when I try to exit a bash shell sometimes. Here is a reproducible scenario in python 2.x: ctrl+c is handled by the interpreter as an exception. ctrl+z 'stops'...
152
votes
9answers
103k views

How to determine where an environment variable came from?

I have a Linux instance that I set up some time ago. When I fire it up and log in as root there are some environment variables that I set up but I can't remember or find where they came from. I've ...
149
votes
10answers
59k views

Is there a one-liner that allows me to create a directory and move into it at the same time?

I find myself repeating a lot of: mkdir longtitleproject cd longtitleproject Is there a way of doing it in one line without repeating the directory name? I'm on bash here.
149
votes
1answer
18k views

Bash: What does “>|” do?

I have just seen this written down; $ some-command >| /tmp/output.txt Vertical pipes are used in standard redirects "piping" the output of one command to another, is >| in fact completely ...
148
votes
14answers
151k views

Batch renaming files

I have a directory full of images: image0001.png image0002.png image0003.png ... And I would like a one-liner to rename them to (say). 0001.png 0002.png 0003.png ... How do I do this?
143
votes
10answers
179k views

Looping through files with spaces in the names? [duplicate]

I wrote the following script to diff the outputs of two directores with all the same files in them as such: #!/bin/bash for file in `find . -name "*.csv"` do echo "file = $file"; diff $...
142
votes
2answers
170k views

How to suspend and resume processes

In the bash terminal I can hit Control+Z to suspend any running process... then I can type fg to resume the process. Is it possible to suspend a process if I only have it's PID? And if so, what ...
140
votes
3answers
385k views

What is the purpose of .bashrc and how does it work?

I found the .bashrc file and I want to know the purpose/function of it. Also how and when is it used?
139
votes
2answers
862k views

How do I remove a directory and all its contents?

In bash all I know is that rmdir directoryname will remove the directory but only if it's empty. Is there a way to force remove subdirectories?
132
votes
8answers
201k views

How to debug a bash script?

I'm having some problems with some scripts in bash, about errors and unexpected behaviors. I would like to investigate the causes of the problems so I can apply fixes. Is there a way I can turn some ...
129
votes
5answers
157k views

What color codes can I use in my PS1 prompt?

I used several colors in my PS1 prompt such as \033]01;31\] # pink \033]00m\] # white \033]01;36\] # bold green \033]02;36\] # green \033]01;34\] # blue \033]01;33\] # bold yellow Where can I ...
124
votes
5answers
204k views

How to uppercase the command line argument?

I searched SO and found that to uppercase a string following would work str="Some string" echo ${str^^} But I tried to do a similar thing on a command-line argument, which gave me the following ...
124
votes
3answers
100k views

Quoting within $(command substitution) in Bash

In my Bash environment I use variables containing spaces, and I use these variables within command substitution. Unfortunately I cannot find the answer on SE. What is the correct way to quote my ...
124
votes
4answers
75k views

Precedence of the shell logical operators &&, ||

I am trying to understand how the logical operator precedence works in bash. For example, I would have expected, that the following command does not echo anything. true || echo aaa && echo ...
123
votes
1answer
380k views

In a bash script, using the conditional “or” in an “if” statement

This question is a sequel of sorts to my earlier question. The users on this site kindly helped me determine how to write a bash for loop that iterates over string values. For example, suppose that ...
122
votes
2answers
20k views

When was the shellshock (CVE-2014-6271/7169) bug introduced, and what is the patch that fully fixes it?

Some context about the bug: CVE-2014-6271 Bash supports exporting not just shell variables, but also shell functions to other bash instances, via the process environment to (indirect) child ...
120
votes
4answers
75k views

Combined `mkdir` and `cd`? [duplicate]

is there any way (what is the easiest way in bash) to combine the following: mkdir foo cd foo The manpage for mkdir does not describe anything like that, maybe there is a fancy version of mkdir? I ...