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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373
votes
8answers
698k 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 ...
268
votes
14answers
84k 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 ...
242
votes
14answers
31k 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 ...
212
votes
6answers
90k 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 ...
169
votes
16answers
129k views

How to do integer & float calculations, in bash or other languages/frameworks?

Using echo "20+5" literally produces the text "20+5". What command can I use to get the numeric sum, 25 in this case? Also, what's the easiest way to do it just using bash for floating point? For ...
155
votes
3answers
29k views

What does “rc” in .bashrc stand for?

Is it "resource configuration", by any chance?
148
votes
17answers
205k 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.
137
votes
21answers
129k views

How can I get my external IP address in bash?

I need to find my external IP address from a bash script. At the moment I use this function: myip () { lwp-request -o text checkip.dyndns.org | awk '{ print $NF }' } But it depends on ...
133
votes
4answers
31k 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 ...
129
votes
15answers
217k 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 ...
122
votes
1answer
9k 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 ...
121
votes
4answers
136k views

Why do I need a tty to run sudo if I can sudo without a password?

I have configured sudo to run without a password, but when I try to ssh 'sudo Foo", I still get sudo: sorry, you must have a tty to run sudo. Why does this happen and how can I work around it?
108
votes
4answers
46k 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 ...
107
votes
2answers
18k 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 ...
101
votes
4answers
94k 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 ...
100
votes
3answers
58k 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 ...
91
votes
6answers
8k 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 ...
90
votes
3answers
53k 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 ...
89
votes
14answers
103k 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?
89
votes
5answers
66k 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 ...
83
votes
4answers
2k views

Can I somehow add a “&& prog2” to an already running prog1?

Most shells provide functions like && and ; to chain the execution of commands in certain ways. But what if a command is already running, can I still somehow add another command to be executed ...
83
votes
2answers
3k 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 ...
77
votes
2answers
7k views

Have backticks (i.e. `cmd`) in *sh shells been deprecated?

I've seen this comment many times on Unix & Linux as well as on other sites that use the phrasing "backticks have been deprecated", with respect to shells such as Bash & Zsh. Is this ...
74
votes
10answers
111k 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. ...
73
votes
9answers
116k 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?
71
votes
4answers
22k views

What is the difference between “&&” and “;” when chaining commands

What is the difference between the following methods of chaining commands? cmd1; cmd2 cmd1 && cmd2
70
votes
14answers
38k views

How to grep a specific line _and_ the first line of a file?

Assuming a simple grep such as: $ psa aux | grep someApp 1000 11634 51.2 0.1 32824 9112 pts/1 SN+ 13:24 7:49 someApp This provides much information, but as the first line of the ps ...
70
votes
5answers
48k 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 ...
69
votes
9answers
4k views

Remember a half-typed command while I check something

I often find myself in the following position: I've started typing a long command at the bash prompt, but half-way through I find out I need to check something with another command. This is a problem ...
67
votes
8answers
106k 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 ...
67
votes
4answers
8k views

Is there a “.bashrc” equivalent file read by all shells?

Is ~/.bashrc the only place to specify user specific environment variables, aliases, modifications to PATH variable, etc? I ask because it seems that ~/.bashrc seems to be bash-only, but other ...
66
votes
4answers
90k views

How to cut by tab character

I want to use cut command to break input into fields and get the second field. cut -f2 -d' ' <--- How can I tell it to use tab instead of space
66
votes
10answers
81k views

Looping through files with spaces in the names?

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 ...
66
votes
2answers
94k 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 ...
63
votes
3answers
47k views

how can shellshock be exploited over SSH?

Apparently, the shellshock Bash exploit CVE-2014-6271 can be exploited over the network via SSH. I can imagine how the exploit would work via Apache/CGI, but I cannot imagine how that would work over ...
60
votes
5answers
193k views

How to append multiple lines to a file with bash

I am writing a bash script to look for a file if it doesn't exist then create it and append this to it: Host localhost ForwardAgent yes So "line then new line 'tab' then text" I think its a ...
59
votes
2answers
9k views

Why does bash think: 016 + 1 = 15?

Can somebody explain to me why a number with a leading 0 gives this funny behaviour? #!/bin/bash NUM=016 SUM=$((NUM + 1)) echo "$NUM + 1 = $SUM" Will print: 016 + 1 = 15
57
votes
7answers
86k 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
57
votes
16answers
72k views

Is there a way to get the min, max, median, and average of a list of numbers in a single command?

I have a list of numbers in a file, one per line. How can I get the minimum, maximum, median and average values? I want to use the results in a bash script. Although my immediate situation is for ...
57
votes
6answers
160k 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..
56
votes
5answers
52k views

What is the difference between “sort -u” and “sort | uniq”?

Everywhere I see someone needing to get a sorted, unique list, they always pipe to sort | uniq. I've never seen any examples where someone uses sort -u instead. Why not? What's the difference, and why ...
56
votes
5answers
72k views

What does ` (backquote/backtick) mean in bash?

I came across the following command sudo chown `id -u` /somedir And I wonder what is the meaning of the ` symbol. I noticed for instance that while the command above works well the one below does ...
53
votes
11answers
31k 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?
53
votes
6answers
7k views

Can I configure my shell to print STDERR and STDOUT in different colors?

I want to set my terminal up so stderr is printed in a different color than stdout; maybe red. This would make it easier to tell the two apart. Is there a way to configure this in .bashrc? If not, is ...
52
votes
4answers
49k 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?
52
votes
5answers
2k views

cd by just typing the directory's name?

I either had this somewhere 20 years ago or I dreamed about it. Basically: If if type blobblob I get blobblob: command not found Fair enough. I would like it so that when my shell gets those ...
52
votes
2answers
34k views

An easy bash completion tutorial? [closed]

I want to learn how to write bash completion scripts. Which tutorial would you recommend for a newbie?
51
votes
7answers
11k 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.
51
votes
3answers
46k views

Multiline shell script comments - how does this work?

Recently, I stumbled upon a multiline comment type I have never seen before - here is a script example: echo a # : aaa : ddd # echo b This seems to work, hell, even vim syntax-highlights it. What ...
51
votes
6answers
7k views

Is there a way to cd back multiple times in bash? [duplicate]

I often use cd - to go back to where I was. How can I do this multiple times in bash? Or would zsh or some other tool support this?