The shell is unix's command-line interface. You can type commands in a shell interactively, or write scripts to automate tasks.

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344
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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?
35
votes
5answers
3k 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 ...
34
votes
2answers
2k 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?
24
votes
3answers
5k views

How to pass parameters to an alias?

For bash script, I can use "$@" to access arguments. What's the equivalent when I use an alias?
29
votes
4answers
9k 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 ...
31
votes
4answers
11k views

Why is `while IFS= read` used so often, instead of `IFS=; while read..`?

It seems that normal practice would put the setting of IFS outside the while loop in order to not repeat setting it for each iteration... Is this just a habitual "monkey see, monkey do" style, as it ...
37
votes
7answers
6k 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 ...
42
votes
12answers
33k 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 ...
51
votes
5answers
27k 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 ...
55
votes
3answers
4k views

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

When looking for the path to an executable or checking what would happen would you enter a command name in a Unix shell, there's a plethora of different utilities (which, type, command, whence, where, ...
25
votes
9answers
7k 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?
138
votes
5answers
7k 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 ...
54
votes
2answers
13k 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 --.
25
votes
3answers
1k views

Can a bash script be hooked to a file?

I want a script to sleep unless a certain file is modifed/deleted (or a file created in a certain directory, or ...). Can this be achieved in some elegant way? The simplest thing that comes to my mind ...
31
votes
3answers
3k views

How to use `which` on an aliased command?

Like most users, I have a bunch of aliases set up to give a default set of flags for frequently used programs. For instance, alias vim='vim -X' alias grep='grep -E' alias ls='ls -G' The problem is ...
93
votes
14answers
29k 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, e.g. 25 in this case. Also, what's the easiest way to do it just using bash for floating point, ...
27
votes
6answers
9k views

How to test what shell I am using in a terminal?

How to check what shell I am using in a terminal? What is the shell I am using in MacOS?
16
votes
2answers
7k views

How can I make iconv replace the input file with the converted output?

I have a bash script which enumerates through every *.php file in a directory and applies iconv to it. This gets output in STDOUT. Since adding the -o parameter ( in my experience ) actually writes ...
14
votes
2answers
745 views

gnu find and masking the {} for some shells - which?

The man page for gnu find states: -exec command ; [...] The string `{}' is replaced by the current file name being processed everywhere it occurs in the arguments to the command, ...
40
votes
12answers
12k 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?
15
votes
4answers
3k views

Four tasks in parallel… how do I do that?

I have a bunch of PNG images on a directory. I have an application called pngout that I run to compress these images. This application is called by a script I did. The problem is that this script does ...
15
votes
1answer
8k views

How to loop over the lines of a file?

Say I have this file: hello world hello world This program #!/bin/bash for i in $(cat $1); do echo "tester: $i" done outputs tester: hello tester: world tester: hello tester: world I'd ...
21
votes
3answers
7k views

Understanding IFS

The following few threads on this site and StackOverflow were helpful for understanding how IFS works: What is IFS in context of for looping? How to loop over the lines of a file Bash, read line by ...
15
votes
6answers
2k views

Any way to sync directory structure when the files are already on both sides?

I have two drives with the same files, but the directory structure is totally different. Is there any way to 'move' all the files on the destination side so that they match the structure of the ...
6
votes
5answers
997 views

How to clean up file extensions?

I have a directories with .MP3 files which I'd like to change the extensions to .mp3. What's the easiest way to do this? I'm think something along the lines of: find /RootPath -type f -iname "*.mp3" ...
83
votes
9answers
25k 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 ...
46
votes
5answers
1k views

Resources for portable shell programming

What resources exist for portable shell programming? The ultimate answer is to test on all targeted platforms, but that's rarely practical. The POSIX / Single UNIX specification is a start, but it ...
70
votes
6answers
5k views

Why is cd not a program?

I've always wondered why cd isn't a program, but never managed to find the answer. Anyone know why this is the case?
30
votes
5answers
4k views

Timing out in a shell script

I have a shell script that's reading from standard input. In rare circumstances, there will be no one ready to provide input, and the script must time out. In case of timeout, the script must execute ...
10
votes
1answer
471 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 ...
11
votes
3answers
3k views

Make `rm` move to trash

Is there a Linux script / application which, instead of deleting files, moves them to a special “trash” location? I’d like this as a replacement for rm (maybe even aliasing the latter; there are pros ...
12
votes
1answer
449 views

How do ${0##*/} and ${0%/*} work?

I'm quite confused about the following regular expressions I found in a shell script: ${0##*/} ${0%/*} How do they work?
4
votes
2answers
600 views

In `while IFS= read..`, why does IFS have no effect?

I might have something absolutely wrong, but it looks convincing to me, that setting IFS as one of the commands in the pre-do/done list has absolutely no effect. The outer IFS (outside the while ...
8
votes
4answers
4k views

How do I handle switches in a shell script?

Are there some built-in tools that will recognize -x and --xxxx as switches, and not arguments, or do you have to go through all the input variables, test for dashes, and then parse the arguments ...
16
votes
1answer
2k views

Is there a way in bash to redirect output and still have it go to stdout?

Okay. If I wanted to redirect the output of a program to a file, I'd do something like this prog > file If I wanted to redirect both stdout and stderr to that file, then I'd do prog > file ...
9
votes
5answers
807 views

Why do I need to quote variable for if, but not for echo?

I've read that you need double quotes for expanding variables, e.g. if [ -n "$test" ]; then echo '$test ok'; else echo '$test null'; fi will work as expected, while if [ -n $test ]; then echo ...
12
votes
5answers
27k views

How can I read line by line from a variable in bash?

I have a variable which contains multiline output of a command. What's the most effecient way to read the output line by line from the variable? For example: jobs="$(jobs)" if [ "$jobs" ]; then ...
12
votes
5answers
9k views

Show only stderr on screen but write both stdout and stderr to file

How can I use BASH magic to achieve this? I want to only see stderr output on the screen, but I want both stdout and stderr to be written to a file. Clarification: I want both stdout and stderr to ...
16
votes
5answers
2k views

how to glob every hidden file except current and parent directory

I want to glob every hidden file and directory, but not the current (.) and parent directory (..). I am using bash. Observe current behaviour: $ ls -a . .. ...a ...aa ..a ..aa .a .aa .aaa ...
7
votes
6answers
6k views

Shell script for moving oldest files?

How do I write a script for moving just the 20 oldest files from one folder to another? Is there a way to grab the oldest files in a folder?
6
votes
3answers
1k views

How to defer variable expansion

I was wanting to initialize some strings at the top of my script with variables that have no yet been set, such as: str1='I went to ${PLACE} and saw ${EVENT}' str2='If you do ${ACTION} you will ...
3
votes
4answers
3k views

executing a sh script from the cron

I have a test.sh script #!/bin/sh php /home/v/file.php sh /root/x/some.sh when I execute the file as root from command line it works. sh /home/v/test.sh when I set it to crontab -e (is the root ...
3
votes
1answer
997 views

Wrapping a command that includes single and double quotes for another command

I recently learned about watch, but am having trouble making it work with relatively sophisticated commands. For example, I would like to ask watch to run the following command on zsh every three ...
3
votes
2answers
1k views

Is there a way to push shell config information when SSHing to a host?

I know how to set the GNOME-terminals (or xterms!) prompt to green/red regarding the last exit code: vi .bashrc export PROMPT_COMMAND='PS1="` if [[ \$? = "0" ]]; then echo "\\[\\033[0;32m\\]"; else ...
29
votes
5answers
2k views

What are the fundamental differences between the mainstream *NIX shells?

What are the fundamental differences between the mainstream *NIX shells and what scenarios might prompt you to use one over the other? I understand that some of it probably comes down to user ...
27
votes
10answers
8k views

How do I reuse the last output from the command line?

This is a noob question, but I'd like to know how to reuse the last output from the console, ie: pv-3:method Xavier$ python -c "from distutils.sysconfig import get_python_lib; print get_python_lib()" ...
28
votes
4answers
28k views

How do you move all files (including hidden) in a directory to another?

How do I move all files in a directory (including the hidden ones) to another directory? For example, if I have a folder "Foo" with the files ".hidden" and "notHidden" inside, how do I move both ...
23
votes
4answers
1k views

What's the difference between running a program as a daemon and forking it into background with '&'?

What are the practical differences from a sysadmin point of view when deploying services on a unix based system?
37
votes
12answers
4k views

best way to search my shell's history

Is there a better way to search my history file for a command than grep? I do have some idea what the command starts as, but I don't know how far back in the history it is. update: was formerly zsh ...
32
votes
4answers
2k views

What is the difference between a builtin command and one that is not?

Is there any intrinsic difference between a builtin command and another command which can nominally do the same thing? eg. Do builtins get "special" treatement? ... is there less overhead running ...