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Questions tagged [shell]

The shell is Unix's command-line interface. You can type commands in a shell interactively, or write scripts to automate tasks. Use this tag for questions applying to /bin/sh and most compatible shells (ash, bash, ksh, zsh, …). For shell scripts with errors, please check them in http://shellcheck.net before posting here.

1188
votes
9answers
312k views

What is the exact difference between a 'terminal', a 'shell', a 'tty' and a 'console'?

I think these terms almost refer to the same thing, when used loosely: terminal shell tty console What exactly does each of these terms refer to?
585
votes
13answers
1.3m views

How do I grep for multiple patterns with pattern having a pipe character?

I want to find all lines in several files that match one of two patterns. I tried to find the patterns I'm looking for by typing grep (foo|bar) *.txt but the shell interprets the | as a pipe and ...
550
votes
3answers
223k views

Difference between nohup, disown and &

What are the differences between $ nohup foo and $ foo & and $ foo & $ disown
436
votes
4answers
151k views

What does “--” (double-dash) mean? (also known as “bare double dash”)

I have seen -- used in the compgen command. For example: compgen -W "foo bar baz" -- b What is the meaning of the -- in there?
381
votes
13answers
194k views

Turn off buffering in pipe

I have a script which calls two commands: long_running_command | print_progress The long_running_command prints a progress but I'm unhappy with it. I'm using print_progress to make it more nice (...
331
votes
10answers
135k views

How do I delete a file whose name begins with “-” (hyphen a.k.a. dash or minus)?

How do you remove a file whose filename begins with a dash (hyphen or minus) -? I'm ssh'd into a remote OSX server and I have this file in my directory: tohru:~ $ ls -l total 8 -rw-r--r-- 1 me ...
312
votes
4answers
39k views

Why not use “which”? What to use then?

When looking for the path to an executable or checking what would happen if you enter a command name in a Unix shell, there's a plethora of different utilities (which, type, command, whence, where, ...
311
votes
11answers
322k views

How do I use pushd and popd commands?

What are the practical uses of both pushd and popd when there is an advantage of using these two commands over cd and cd -? EDIT: I'm looking for some practical examples of uses for both of these ...
291
votes
3answers
1.6m views

How to check OS and version using a Linux command [duplicate]

What is the Linux command to check the server OS and its version? I am connected to the server using shell.
291
votes
5answers
158k views

Difference between Login Shell and Non-Login Shell?

I understand the basic difference between an interactive shell and a non-interactive shell. But what exactly differentiates a login shell from a non-login shell? Can you give examples for uses of a ...
285
votes
16answers
339k 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 ...
275
votes
10answers
685k views

How to append multiple lines to a file

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 ...
268
votes
16answers
127k views

Get exit status of process that's piped to another

I have two processes foo and bar, connected with a pipe: $ foo | bar bar always exits 0; I'm interested in the exit code of foo. Is there any way to get at it?
265
votes
4answers
30k views

How does a Segmentation Fault work under-the-hood?

I can't seem to find any information on this aside from "the CPU's MMU sends a signal" and "the kernel directs it to the offending program, terminating it". I assumed that it probably sends the ...
264
votes
4answers
207k 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 ...
262
votes
6answers
25k views

What's the difference between $(stuff) and `stuff`?

There are two syntaxes for command substitution: with dollar-parentheses and with backticks. Running top -p $(pidof init) and top -p `pidof init` gives the same output. Are these two ways of doing the ...
255
votes
21answers
643k 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
467k views

How to split the terminal into more than one “view”?

From vi, if you issue the command :sp, the screen splits into two "views", allowing you to edit more than one file from the same terminal. Along those same lines, is there a way to have multiple ...
240
votes
5answers
285k 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 ...
235
votes
8answers
459k 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 ...
233
votes
3answers
94k views

What are the shell's control and redirection operators?

I often see tutorials online that connect various commands with different symbols. For example: command1 | command2 command1 & command2 command1 || command2 command1 && command2 ...
223
votes
14answers
399k 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?
217
votes
6answers
79k 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 ...
205
votes
6answers
200k views

How to go to the previous working directory in terminal?

In terminal, how can I define a key to go to the previous directory which I was in when changing directory with the cd command? For example, I'm in /opt/soft/bin and I cd into /etc/squid3 and I want ...
199
votes
8answers
32k views

Why *not* parse `ls` (and what do to instead)?

I consistently see answers quoting this link stating definitively "Don't parse ls!" This bothers me for a couple of reasons: It seems the information in that link has been accepted wholesale with ...
199
votes
3answers
16k 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 ...
189
votes
7answers
196k 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?
187
votes
4answers
35k views

Why is using a shell loop to process text considered bad practice?

Is using a while loop to process text generally considered bad practice in POSIX shells? As Stéphane Chazelas pointed out, some of the reasons for not using shell loop are conceptual, reliability, ...
185
votes
20answers
226k views

How can I get distribution name and version number in a simple shell script?

I'm working on a simple bash script that should be able to run on Ubuntu and CentOS distributions (support for Debian and Fedora/RHEL would be a plus) and I need to know the name and version of the ...
183
votes
21answers
252k 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?
182
votes
8answers
135k views

Allow setuid on shell scripts

The setuid permission bit tells Linux to run a program with the effective user id of the owner instead of the executor: > cat setuid-test.c #include <stdio.h> #include <unistd.h> int ...
182
votes
3answers
163k views

Difference between 2>&-, 2>/dev/null, |&, &>/dev/null and >/dev/null 2>&1

Just looking for the difference between 2>&- 2>/dev/null |& &>/dev/null >/dev/null 2>&1 and their portability with non-Bourne shells like tcsh, mksh, etc.
175
votes
8answers
125k views

How can I send stdout to multiple commands?

There are some commands which filter or act on input, and then pass it along as output, I think usually to stdout - but some commands will just take the stdin and do whatever they do with it, and ...
171
votes
12answers
248k views

How do I convert an epoch timestamp to a human readable format on the cli?

How do I convert an epoch timestamp to a human readable format on the cli? I think there's a way to do it with date but the syntax eludes me (other ways welcome).
164
votes
7answers
451k 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..
162
votes
7answers
90k views

Prepending a timestamp to each line of output from a command

I wish to prepend a timestamp to each line of output from a command. For example: foo bar baz would become [2011-12-13 12:20:38] foo [2011-12-13 12:21:32] bar [2011-12-13 12:22:20] baz ...where ...
160
votes
5answers
365k views

How to suspend and bring a background process to foreground

I have a process originally running in the foreground. I suspended by Ctrl+Z, and then resume its running in the background by bg <jobid>. I wonder how to suspend a process running in the ...
160
votes
4answers
122k views

Find out current working directory of a running process?

What command(s) can one use to find out the current working directory (CWD) of a running process? These would be commands you could use externally from the process.
160
votes
8answers
193k views

What's the difference between /sbin/nologin and /bin/false

Technically, unless pam is set up to check your shell with pam_shells neither of these can actually prevent your login, if you're not on the shell. On my system they are even different sizes, so I ...
159
votes
6answers
32k 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 ...
157
votes
5answers
523k 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 ...
152
votes
9answers
275k views

How to monitor CPU/memory usage of a single process?

I would like to monitor one process's memory / cpu usage in real time. Similar to top but targeted at only one process, preferably with a history graph of some sort.
151
votes
6answers
250k views

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

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 ...
151
votes
2answers
236k 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'...
148
votes
10answers
179k 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 ...
148
votes
14answers
144k 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?
145
votes
7answers
366k views

How to overwrite target files with mv?

I have a ton of files and dirs in a subdirectory I want to move to the parent directory. There are already some files and dirs in the target directory which need to be overwritten. Files that are only ...
142
votes
3answers
62k views

What's the difference between semicolon and double ampersand && [duplicate]

What is the difference between echo "Hello " ; echo "world" and echo "Hello " && echo "world" Both seems to run the two commands after each other.
141
votes
10answers
56k 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.
141
votes
9answers
97k 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 ...