The shell is unix's command-line interface. You can type commands in a shell interactively, or write scripts to automate tasks.
232
votes
4answers
41k 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 do each of these terms refer to?
23
votes
4answers
2k 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 ...
17
votes
4answers
5k 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 ...
23
votes
2answers
1k views
$VAR vs ${VAR} and to quote or not to quote
I can write
VAR=$VAR1
VAR=${VAR1}
VAR="$VAR1"
VAR="${VAR1}"
the end result to me all seems about the same. Why should I write one or the other? are any of these not portable/POSIX?
16
votes
3answers
4k views
How to pass parameters to an alias?
For bash script, I can use "$@" to access arguments. What's the equivalent when I use an alias?
28
votes
11answers
20k 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 ...
31
votes
5answers
17k 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 ...
30
votes
2answers
6k 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 --.
26
votes
7answers
2k 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 ...
22
votes
3answers
1k 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 ...
18
votes
5answers
5k 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
7answers
4k 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?
13
votes
2answers
4k 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 ...
23
votes
10answers
5k 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?
19
votes
2answers
832 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 ...
11
votes
3answers
2k 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 ...
100
votes
3answers
5k views
What's the difference between $(stuff) and `stuff`?
Running top -p $(pidof init) and top -p `pidof init` gives the same output. Are these two ways of doing the same thing, or are there differences?
43
votes
5answers
845 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 ...
12
votes
4answers
1k 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 ...
11
votes
2answers
481 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, ...
4
votes
5answers
835 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" ...
13
votes
2answers
2k 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 ...
14
votes
1answer
1k 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 ...
10
votes
1answer
376 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?
75
votes
13answers
10k views
How to do integer & float calculations, in bash or other languages?
Using echo "20+5" literally produces 20+5
What command can I use to get the actual sum, e.g. 25 in this case.
Also, what's the easiest way to do it just using bash for floating point, e.g.
echo ...
47
votes
8answers
12k 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 ...
26
votes
4answers
3k 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 ...
57
votes
6answers
4k 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?
12
votes
1answer
3k 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: $i"
done
outputs
tester: hello
tester: world
tester: hello
tester: world
I'd ...
9
votes
6answers
14k views
How can I read line by line from a variable in bash?
I have a variable which contains multiline output of a command. What's the most effecient way to read the output line by line from the variable?
For example:
jobs="$(jobs)"
if [ "$jobs" ]; then
...
13
votes
6answers
1k views
Any way to sync directory structure when the files are already on both sides?
I have two drives with the same files, but the directory structure is totally different.
Is there any way to 'move' all the files on the destination side so that they match the structure of the ...
8
votes
3answers
1k views
Make `rm` move to trash
Is there a Linux script / application which, instead of deleting files, moves them to a special “trash” location? I’d like this as a replacement for rm (maybe even aliasing the latter; there are pros ...
6
votes
6answers
4k views
Shell script for moving oldest files?
How do I write a script for moving just the 20 oldest files from one folder to another? Is there a way to grab the oldest files in a folder?
6
votes
4answers
2k views
How do I handle switches in a shell script?
Are there some built-in tools that will recognize -x and --xxxx as switches, and not arguments, or do you have to go through all the input variables, test for dashes, and then parse the arguments ...
30
votes
12answers
2k views
best way to search my shell's history
Is there a better way to search my history file for a command than grep? I do have some idea what the command starts as, but I don't know how far back in the history it is.
update: was formerly zsh ...
22
votes
5answers
2k views
What are the fundamental differences between the mainstream *NIX shells?
What are the fundamental differences between the mainstream *NIX shells and what scenarios might prompt you to use one over the other? I understand that some of it probably comes down to user ...
17
votes
10answers
4k views
How do I reuse the last output from the command line?
This is a noob question, but I'd like to know how to reuse the last output from the console, ie:
pv-3:method Xavier$ python -c "from distutils.sysconfig import get_python_lib; print get_python_lib()"
...
11
votes
5answers
5k 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 ...
7
votes
3answers
840 views
Can I make `cut` change a file in place?
The man page doesn't give me much hope, but I'm hoping it's an undocumented (and/or GNU-specific) feature.
6
votes
5answers
311 views
IO redirection and the head command
I was trying to quickly edit an .hgignore file from the Cygwin bash shell today, and I added a line that was a mistake. I'm not sure if this was the best way to do it, but I quickly thought of using ...
5
votes
1answer
630 views
Best way to remove file extension from a string?
So, I'm using a script I've made to convert videos to the webm format. A certain program calls the script, sending %f which is the full, absolute file name of the video, like this:
converter.sh %f
...
3
votes
2answers
368 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 ...
6
votes
5answers
4k views
sh startup files over ssh
I have some important commands I need to execute before any sh shell starts. This is required for passing SSH commands in the SSH command (ssh host somecommand) and other programs that run commands.
...
2
votes
2answers
904 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 ...
21
votes
1answer
337 views
A shell tool to “tablify” input data
A long time ago I remember using a command that makes its input into a nicely formatted table.
For example, for this input,
apple 1 100
orange 20 19
pineapple 1000 87
avocado 4 30
The output will ...
14
votes
3answers
12k 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 ...
10
votes
4answers
541 views
keep duplicates out of $PATH on source
I have the following code that's source-d by my .shellrc
PATH="${PATH}:${HOME}/perl5/bin"
PATH="${PATH}:${HOME}/.bin"
export PATH
but if I make changes to other code and then source this file, my ...
13
votes
2answers
1k views
How to apply recursively chmod directories without affecting files?
After I apply chmod -R to a directory, permissions are changed for everything within (files and directories). How can I add execute/search (x) permissions to directories without modifying the files?
8
votes
1answer
207 views
Order of redirections
I don't quite understand how the computer reads this command.
cat file1 file2 1> file.txt 2>&1
If I understand, 2>&1 simply redirect Standard Error to Standard Output.
By that ...
5
votes
3answers
1k views
What is IFS in context of for looping?
I'm learning bash scripting right now.
I was reading this thread: How to loop over the lines of a file?
What is IFS?
With for and IFS:
