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.
11,986
questions
1552
votes
10
answers
487k
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?
- 34.7k
799
votes
13
answers
1.8m
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 ...
- 9,432
795
votes
5
answers
346k
views
What does "--" (double-dash) mean?
I have seen -- used in the compgen command.
For example:
compgen -W "foo bar baz" -- b
What is the meaning of the -- in there?
- 29.3k
778
votes
5
answers
397k
views
Difference between nohup, disown and &
What are the differences between
$ nohup foo
and
$ foo &
and
$ foo &
$ disown
- 27.2k
621
votes
11
answers
1.3m
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 ...
- 6,655
553
votes
8
answers
314k
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 ...
- 7,399
546
votes
7
answers
276k
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 ...
- 5,609
529
votes
15
answers
316k
views
Turn off buffering in pipe
I have a script which calls two commands:
long_running_command | print_progress
The long_running_command prints progress but I'm unhappy with it. I'm using print_progress to make it nicer (namely, I ...
- 5,988
497
votes
19
answers
257k
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?
- 92.1k
486
votes
10
answers
207k
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 ...
- 4,963
480
votes
5
answers
88k
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, ...
- 532k
470
votes
11
answers
482k
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 ...
- 12.2k
423
votes
3
answers
211k
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
...
- 238k
416
votes
17
answers
627k
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 ...
- 41.6k
412
votes
13
answers
255k
views
How can I find broken symlinks
Is there a way to find all symbolic links that don't point anywere?
find ./ -type l
will give me all symbolic links, but makes no distinction between links that go somewhere and links that don't. ...
- 11.7k
407
votes
13
answers
696k
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?
- 4,231
397
votes
3
answers
1.9m
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.
- 4,081
383
votes
13
answers
408k
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?
- 7,729
372
votes
6
answers
398k
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 ...
365
votes
5
answers
577k
views
How do I set an environment variable on the command line and have it appear in commands?
If I run
export TEST=foo
echo $TEST
It outputs foo.
If I run
TEST=foo echo $TEST
It does not. How can I get this functionality without using export or a script?
- 4,307
364
votes
12
answers
223k
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 ...
anon
338
votes
4
answers
432k
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 ...
- 5,559
331
votes
10
answers
714k
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 ...
- 16.7k
326
votes
5
answers
1.2m
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 ...
user104976
319
votes
12
answers
574k
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 ...
- 7,031
317
votes
10
answers
812k
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..
- 39.7k
312
votes
25
answers
950k
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 ...
- 19.5k
308
votes
16
answers
477k
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.
- 3,896
292
votes
7
answers
338k
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 ...
- 5,036
290
votes
6
answers
44k
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 ...
- 64.9k
286
votes
12
answers
438k
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).
- 58.5k
283
votes
10
answers
205k
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 ...
- 44.9k
283
votes
8
answers
209k
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 ...
- 2,933
281
votes
4
answers
41k
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 ...
- 2,203
280
votes
7
answers
232k
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.
slm♦
- 366k
278
votes
19
answers
450k
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?
- 15.2k
274
votes
13
answers
323k
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 ...
- 3,663
271
votes
5
answers
70k
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, ...
- 152k
271
votes
5
answers
599k
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 ...
- 100k
269
votes
4
answers
50k
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 ...
- 532k
267
votes
10
answers
66k
views
Why *not* parse `ls` (and what to do 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 ...
- 57.8k
259
votes
11
answers
220k
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 ...
- 92.1k
255
votes
6
answers
474k
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 ...
- 2,661
254
votes
3
answers
227k
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.
- 2,662
243
votes
2
answers
443k
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'...
- 4,362
239
votes
8
answers
332k
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 ...
- 58.5k
239
votes
3
answers
118k
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.
- 4,156
238
votes
7
answers
648k
views
How to move and overwrite subdirectories (and files) to parent directory?
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 ...
- 2,813
226
votes
3
answers
147k
views
Are there naming conventions for variables in shell scripts?
Most languages have naming conventions for variables, the most common style I see in shell scripts is MY_VARIABLE=foo. Is this the convention or is it only for global variables? What about variables ...
- 5,211
222
votes
6
answers
449k
views
Can I redirect output to a log file and background a process at the same time?
Can I redirect output to a log file and a background process at the same time?
In other words, can I do something like this?
nohup java -jar myProgram.jar 2>&1 > output.log &
Or, is ...
- 4,087