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352 votes
6 answers
355k 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 ...
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253 votes
4 answers
60k 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, ...
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  • 143k
262 votes
9 answers
55k 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 ...
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  • 55.6k
655 votes
4 answers
287k views

Why is printf better than echo?

I have heard that printf is better than echo. I can recall only one instance from my experience where I had to use printf because echo didn't work for feeding some text into some program on RHEL 5.8 ...
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  • 11.6k
156 votes
1 answer
67k views

When is double-quoting necessary?

The old advice used to be to double-quote any expression involving a $VARIABLE, at least if one wanted it to be interpreted by the shell as one single item, otherwise, any spaces in the content of $...
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  • 12.8k
255 votes
3 answers
40k 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 ...
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382 votes
3 answers
185k 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 ...
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  • 215k
203 votes
8 answers
31k views

Why is looping over find's output bad practice?

This question is inspired by Why is using a shell loop to process text considered bad practice ? I see these constructs for file in `find . -type f -name ...`; do smth with ${file}; done and for ...
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1491 votes
10 answers
447k 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?
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  • 33k
124 votes
1 answer
87k views

Understanding the -exec option of `find`

I find myself constantly looking up the syntax of find . -name "FILENAME" -exec rm {} \; mainly because I don't see how exactly the -exec part works. What is the meaning of the braces, the ...
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103 votes
1 answer
16k views

Why does my regular expression work in X but not in Y?

I wrote a regular expression which works well in a certain program (grep, sed, awk, perl, python, ruby, ksh, bash, zsh, find, emacs, vi, vim, gedit, …). But when I use it in a different program (or on ...
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131 votes
5 answers
171k views

How can we run a command stored in a variable?

$ ls -l /tmp/test/my\ dir/ total 0 I was wondering why the following ways to run the above command fail or succeed? $ abc='ls -l "/tmp/test/my dir"' $ $abc ls: cannot access '"/tmp/test/my': No ...
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  • 89.8k
446 votes
4 answers
75k 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, ...
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132 votes
4 answers
12k views

Why are there so many different ways to measure disk usage?

When I sum up the sizes of my files, I get one figure. If I run du, I get another figure. If I run du on all the files on my partition, it doesn't match what df claims is used. Why are there so many ...
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477 votes
9 answers
354k views

Execute vs Read bit. How do directory permissions in Linux work?

In my CMS, I noticed that directories need the executable bit (+x) set for the user to open them. Why is the execute permission required to read a directory, and how do directory permissions in Linux ...
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  • 5,833
491 votes
14 answers
282k 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 ...
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389 votes
17 answers
547k 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 ...
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131 votes
4 answers
63k 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 ...
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  • 23.5k
226 votes
15 answers
486k views

How to run a specific program as root without a password prompt?

I need to run something as sudo without a password, so I used visudo and added this to my sudoers file: MYUSERNAME ALL = NOPASSWD: /path/to/my/program Then I tried it out: $ sudo /path/to/my/...
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  • 36.1k
106 votes
3 answers
337k views

How can I assign the output of a command to a shell variable?

I want to assign the result of an expression (i.e., the output from a command) to a variable and then manipulate it – for example, concatenate it with a string, then echo it.  Here's what I've got: #!/...
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  • 1,208
93 votes
3 answers
168k views

Open a window on a remote X display (why "Cannot open display")?

Once upon a time, DISPLAY=:0.0 totem /path/to/movie.avi after ssh 'ing into my desktop from my laptop would cause totem to play movie.avi on my desktop. Now it gives the error: No protocol ...
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  • 1,251
19 votes
2 answers
9k views

Brackets in if condition: why am I getting syntax errors without whitespace?

I am using the below script to move two days back when script runs at starting two days of the year and also check first and second days of every month and move two days back. if [$month="01"...
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  • 331
97 votes
4 answers
110k views

Understanding "IFS= read -r line"

I obviously understand that one can add value to internal field separator variable. For example: $ IFS=blah $ echo "$IFS" blah $ I also understand that read -r line will save data from stdin to ...
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  • 6,278
486 votes
7 answers
272k 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 ...
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  • 6,499
238 votes
11 answers
195k 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 ...
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129 votes
7 answers
23k views

Redirecting stdout to a file you don't have write permission on

When you attempt to modify a file without having write permissions on it, you get an error: > touch /tmp/foo && sudo chown root /tmp/foo > echo test > /tmp/foo zsh: permission denied:...
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183 votes
9 answers
141k views

How can I disown a running process and associate it to a new screen shell?

I have a running program on a SSH shell. I want to pause it and be able to unpause its execution when I come back. One way I thought of doing that was to transfer its ownership to a screen shell, ...
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  • 3,465
31 votes
3 answers
27k views

Why is my variable local in one 'while read' loop, but not in another seemingly similar loop?

Why do I get different values for $x from the snippets below? #!/bin/bash x=1 echo fred > junk ; while read var ; do x=55 ; done < junk echo x=$x # x=55 .. I'd expect this result x=1 cat ...
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  • 31k
418 votes
15 answers
80k 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 ...
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  • 12.8k
453 votes
10 answers
191k 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 ...
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  • 4,633
113 votes
2 answers
29k views

How do keyboard input and text output work?

Suppose I press the A key in a text editor and this inserts the character a in the document and displays it on the screen. I know the editor application isn't directly communicating with the hardware (...
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66 votes
9 answers
262k views

How can I use variables in the LHS and RHS of a sed substitution?

I want to do: cat update_via_sed.sh | sed 's/old_name/new_name/' > new_update_via_sed.sh in my program. But I want to use variables, e.g. old_run='old_name_952' new_run='old_name_953' I have ...
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145 votes
3 answers
31k 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 ...
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  • 344k
76 votes
3 answers
113k views

How do I run 32-bit programs on a 64-bit Debian/Ubuntu?

I have a 64-bit (amd64 a.k.a. x86_64) Debian or Ubuntu installation. I need to run 32-bit (i386/i686) programs occasionally, or to compile programs for a 32-bit system. How can I do this with a ...
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27 votes
1 answer
7k views

How to ensure that string interpolated into `sed` substitution escapes all metachars

I have a script that reads a text stream and generates a file of sed commands that is later run with sed -f. The generated sed commands are like: s/cid:image002\.gif@01CC3D46\.926E77E0/https:\/\/...
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  • 3,777
1239 votes
12 answers
3.0m 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 ...
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  • 15.6k
190 votes
5 answers
86k views

$VAR vs ${VAR} and to quote or not to quote

I can write VAR=$VAR1 VAR=${VAR1} VAR="$VAR1" VAR="${VAR1}" the end result to me all seems about the same. Why should I write one or the other? are any of these not portable/POSIX?
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77 votes
2 answers
60k views

Getting "Not found" message when running a 32-bit binary on a 64-bit system

I have currently a strange problem on debian (wheezy/amd64). I have created a chroot to install a server (i can't give any more detail about it, sorry). Let's call its path /chr_path/. To make things ...
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  • 949
42 votes
1 answer
12k views

Is it possible to use `find -exec sh -c` safely?

I'm trying to use find to echo 0 into some files, but apparently this only works with sh -c: find /proc/sys/net/ipv6 -name accept_ra -exec sh -c 'echo 0 > {}' \; But using sh -c with find -exec ...
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5 votes
1 answer
16k views

Wi-Fi problems using ASUS USB-N13 adapter (realtek)

I have recently bought an ASUS Wi-Fi adapter (USB-N13), and I seem to be having some trouble with it. Every so often, the Wi-Fi connection just stops working, but the indicator says that I'm still ...
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  • 1,678
709 votes
5 answers
282k 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?
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  • 26.9k
89 votes
6 answers
74k views

Understanding UNIX permissions and file types

I've never really got how chmod worked up until today. I followed a tutorial that explained a big deal to me. For example, I've read that you've got three different permission groups: owner (u) ...
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  • 961
913 votes
10 answers
1.8m views

How can I replace a string in a file(s)?

Replacing strings in files based on certain search criteria is a very common task. How can I replace string foo with bar in all files in the current directory? do the same recursively for sub ...
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  • 215k
368 votes
4 answers
622k views

What characters do I need to escape when using sed in a sh script?

Take the following script: #!/bin/sh sed 's/(127\.0\.1\.1)\s/\1/' [some file] If I try to run this in sh (dash here), it'll fail because of the parentheses, which need to be escaped. But I don't ...
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  • 4,570
673 votes
8 answers
186k views

What do the numbers in a man page mean?

So, for example, when I type man ls I see LS(1). But if I type man apachectl I see APACHECTL(8) and if I type man cd I end up with cd(n). I'm wondering what the significance of the numbers in the ...
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  • 6,857
80 votes
2 answers
19k views

When is dd suitable for copying data? (or, when are read() and write() partial)

Short version: In what circumstances is dd safe to use for copying data, safe meaning that there is no risk of corruption due to a partial read or write? Long version — preamble: dd is often used to ...
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281 votes
11 answers
276k views

Keep processes running after SSH session disconnects

I sometimes have long running processes that I want to kick off before going home, so I create a SSH session to the server to start the process, but then I want to close my laptop and go home and ...
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183 votes
6 answers
163k views

How can I execute `date` inside of a cron tab job?

I want to create a log file for a cron script that has the current hour in the log file name. This is the command I tried to use: 0 * * * * echo hello >> ~/cron-logs/hourly/test`date "+%d"`.log ...
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  • 41.3k
510 votes
3 answers
420k views

What is a bind mount?

What is a “bind mount”? How do I make one? What is it good for? I've been told to use a bind mount for something, but I don't understand what it is or how to use it.
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87 votes
5 answers
61k views

Is there a way to determine the optimal value for the bs parameter to dd?

On occasion I've seen comments online along the lines of "make sure you set 'bs=' because the default value will take too long," and my own extremely-unscientific experiences of, "well that seemed to ...
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