Questions about shell scripts, executable files that are interpreted by a shell (bash, zsh, etc.).

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12
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5answers
18k views

Delete range of lines above pattern with sed (or awk)

I have the following code that will remove lines with the pattern banana and 2 lines after it: sed '/banana/I,+2 d' file So far, so good! But I need it to remove 2 lines before banana, but I can't ...
12
votes
3answers
7k views

Add thousands separator in a number

In python re.sub(r"(?<=.)(?=(?:...)+$)", ",", stroke ) To split a number by triplets, e.g.: echo 123456789 | python -c 'import sys;import re; print re.sub(r"(?<=.)(?=(?:...)+$)", ",", ...
12
votes
2answers
4k views

Piping data to a process's stdin without causing EOF afterward

I have an executable that starts a user-interactive shell. I would like to, upon launch of the shell, inject a few commands first, then allow the user to have their interactive session. I can do this ...
12
votes
3answers
52k views

Concatenating two variables with an underscore

I need to concatenate two variables to create a filename that has an underscore. Lets call my variables $FILENAME and $EXTENSION where filename is read from a file. FILENAME=Hello EXTENSION=WORLD.txt ...
12
votes
7answers
9k views

Print shell arguments in reverse order

I am a bit stuck. My task is to print the arguments to my script in reverse order except the third and fourth. What I have is this code: #!/bin/bash i=$# for arg in "$@" do case $i in ...
12
votes
2answers
27k views

In a bash shell script, writing a for loop that iterates over string values

In bash, I know that it is possible to write a for loop in which some loop control variable i iterates over specified integers. For example, I can write a bash shell script that prints the integers ...
12
votes
3answers
1k views

How do I wait on a program started in another shell

I have a program that does a large a amount of work (takes about 4-5 hours) that gets started by cron when all the data it works with becomes available. Sometimes, when I am waiting for it to finish, ...
12
votes
2answers
2k views

Suppress output unless non-zero exit code

What's the best way to suppress output (stdout and stderr) unless the program exits with a non-zero code? I'm thinking: quiet_success() { file=$(mktemp) if ! ( "$@" > "$file" 2>&1 ); ...
12
votes
3answers
1k views

Why does set -e not work inside () ||

I've run across some scripting like this recently: ( set -e ; do-stuff; do-more-stuff; ) || echo failed This looks fine to me, but it does not work! The set -e does not apply, when you add the ||. ...
12
votes
1answer
2k views

what is the zsh equivalent of bash's export -f

So I started using zsh. I like it all right. It seems very cool and slick, and the fact that the current working directory and actual command line are on different lines is nice, but at the same time, ...
12
votes
1answer
186 views

POSIX equivalent for GNU timeout?

The GNU coreutils timeout command is extremely handy for certain scripting situations, allowing for using the output of a command if it is quick to run, and skipping it if it would take too long. How ...
12
votes
0answers
359 views

Should I use $* or $@? [duplicate]

Which one of $* or $@ is better to use (in array syntax,command line parameters, etc.) to avoid any bugs/problems in code? Or it does not make any difference?
11
votes
3answers
112k views

How to get IP Address using shell script? [duplicate]

Want to get the IP address using Shell script. Without knowing the eth0 or eth1 or eth2 How to get the particular IP address. I am not interest to get localhost address, want to get private IP ...
11
votes
5answers
15k views

generate consistent machine unique ID

Can we generate a unique id for each PC, something like uuuidgen, but it will never change unless there are hardware changes? I was thinking about merging CPUID and MACADDR and hash them to generate ...
11
votes
3answers
755 views

How do you remove dot character from string without calling sed or awk again?

I have a file called hostlist.txt that contains text like this: host1.mydomain.com host2.mydomain.com anotherhost www.mydomain.com login.mydomain.com somehost host3.mydomain.com I have the ...
11
votes
4answers
2k views

How can measure how long it takes my script to run AND include that in an email it generates?

I have a simple bash script that runs a series of checks (ping, nslookup, etc) and then sends an email report with the output of that data. I'd like the email to include information on how long it ...
11
votes
7answers
32k views

Calling multiple bash scripts and running them in parallel, not in sequence

Suppose that I have three (or more) bash scripts: script1.sh, script2.sh, and script3.sh. I would like to call all three of these scripts and run them in parallel. One way to do this is to just ...
11
votes
2answers
18k views

Bash scripting - loop until return value is 0

I need to umount something in my script, but sometimes it unmount's before all of the data has finished being copied and causes the umount to fail. I looked for a way to do a "blocking" umount, but I ...
11
votes
7answers
4k views

Shortest way to extract last 3 characters of base (minus suffix) filename

I'm trying to set a variable in an sh script to the last 3 characters of the base name of a file (by base name I mean without the path and without the suffix). I've succeeded in doing this but, purely ...
11
votes
4answers
719 views

Fast method of splitting string from textfile?

I have a two text files: string.txt and lengths.txt String.txt: abcdefghijklmnopqrstuvwxyz lengths.txt 5 4 10 7 I want to get the file >Entry_1 abcde >Entry_2 fghi >Entry_3 jklmnopqrs ...
11
votes
2answers
1k views

date - years prior to 1901 are treated as invalid

I have date (GNU coreutils) 7.1 installed on my system. If I try to check dates prior to 14-Dec-1901, I get an "invalid date" error. For example, $ date -d 1901-12-13 date: invalid date `1901-12-...
11
votes
4answers
8k views

exit shell script from a subshell

Consider this snippet: stop () { echo "${1}" 1>&2 exit 1 } func () { if false; then echo "foo" else stop "something went wrong" fi } Normally when func ...
11
votes
4answers
7k views

How can I receive top-like CPU statistics from the shell?

I'm trying to get an accurate read of my used CPU (in percent) from top. This is the command I'm running for testing: top -n1 | awk '/Cpu\(s\):/ {print $2}' This returns: 10.7%us, Which is the ...
11
votes
1answer
1k views

How to generate a lot of (blank) images files?

For a test, I need to quickly create 1700 jpg-images (even blank) in a defined size 1000x706. I wonder if we could do this with a shell script or a program (like ImageMagick). I wish they called img_0....
11
votes
1answer
558 views

Is there any way to tell if a shell script was killed with signal 9

I have a bash script (called from a java program) that keeps getting killed. I always catch the signal 15 with trap but then some other signal comes along that I suspect is signal 9 but am in ...
11
votes
5answers
8k views

Exit Google Chrome from terminal

Is there a way to cause google-chrome to quit, from the terminal, besides using killall google-chrome? I would like to be able to close it from a script without killing it.
11
votes
3answers
4k views

What does `1>>` and `2>>` mean in a bash script?

I have the following bash script, from what I understand >> is used to append the output of a command to an existing file instead of overwrite, but what is it doing in this case? This script is ...
11
votes
3answers
9k views

Diff, show modified line from right file only

I have file a and b and I would like to output lines of b that changed since it was cloned from a. Just the modified lines, no surrounding context, no diff offset marks. How can I do that using shell ...
11
votes
2answers
12k views

Regular expression in bash script

This is my first time bash scripting so I'm probably making an easy mistake. Basically, I'm trying to write a script that gets the groups of a user, and if they are in a certain group, it will log ...
11
votes
1answer
196 views

Why do these bash fork bombs work differently and what is the significance of & in it?

I get how a normal fork bomb works, but I don't really understand why the & at the end of the common bash fork bomb is required and why these scripts behave differently: :(){ (:) | (:) }; : and ...
11
votes
5answers
18k views

Print Matching line and nth line from the matched line

I am trying to print the matched line and the 4th line from the matched line (line containing the expression I am searching for). I have been using the following code: sed -n 's/^[ \t]*//; /img class=...
11
votes
2answers
4k views

Using sed to color the output from a command on solaris

I have a ksh script that must work on both linux and solaris. I'm trying to color the output of specific commands. It works on linux (specifically RHEL6), but not on solaris (SunOS 5.10). Command ...
11
votes
1answer
2k views

Know which process does I/O without iotop

On Linux, I want to know which process uses my disk I/O bandwith. I know I can use iotop but I have a machine where, for various reasons, iotop cannot be installed. How can I manually get that ...
11
votes
2answers
377 views

How to start tailing a file that has not been yet created

I use tail to monitor the progress of jobs that I know will write their progress to disk. Almost always, I know which file they will create before they start running (the jobs are dispatched by a ...
11
votes
3answers
1k views

Are free shell accounts safe?

I am trying to identify how safe the free shell accounts provided by the likes of shellmix.com are. I have never used one myself but want to suggest it to someone interested in learning shell ...
11
votes
3answers
3k views

Is there a way to convert a zip to a tar without extracting it to the filesystem?

Is there a way to convert a zip archive to a tar archive without extracting to a temporary directory first? (and without writing my own implementation of tar or unzip)
11
votes
4answers
15k views

find -exec in bash script with variable expansion

I'm trying to run a command similar to the one below in a bash script. It should search through all subfolders of $sourcedir and copy all files of a certain type to the root level of $targetdir. #!/...
11
votes
4answers
388 views

Get the display width of a string of characters

What would be the closest to a portable way to get the display width (on a terminal at least (one that displays characters in the current locale with the correct width)) of a string of characters from ...
11
votes
3answers
27k views

How to set global environment variables at boot through a script, and have them available for an application that runs before login?

I have a service that runs at boot, and in that service it calls a bash script in the background that exports some environment variables. The problem I'm having is that those environment variables ...
11
votes
1answer
2k views

How do I make a shell script that sends output to a process

I'm currently running a server console program in a screen because I need to both read it and occasionally send commands. I'd like to run the app as a deamon in the background (start/stop it with ...
10
votes
6answers
1k views

Is it possible to substitute “then” with brackets?

I first learned how to code in a C language, and using if ( code ) then code fi makes it really hard for me to read my code (don't ask why). Is there a substitute for then? Something like this: ...
10
votes
6answers
751 views

Remove lines from a file depending on lines found in another file

File file1.txt contains lines like: /api/purchase/<hash>/index.html For example: /api/purchase/12ab09f46/index.html File file2.csv contains lines like: <hash>,timestamp,ip_address ...
10
votes
3answers
13k views

Prompt user to login as root when running a shell script

The problem I am getting is, when I enter the command, su - root at the beginning of my shell script file, it prompts the user to enter the password and then does NOT continue with the rest of the ...
10
votes
3answers
43k views

What does $# mean in shell?

What does $# mean in shell? I have code such as if [ $# -eq 0 ] then I want to understand what $# means, but Google search is very bad for searching these kinds of things.
10
votes
8answers
6k views

cron Vs. sleep - which is the better one in terms of efficient cpu/memory utilization?

The Case: I need to run some commands/script at certain intervals of time and for this I have two options: set up a cron-job implement a loop with sleep in the script itself. Question: Which is ...
10
votes
4answers
9k views

How to run parallel processes and combine outputs when both finished

I have a bash shell script in which I pipe some data through about 5 or 6 different programs then the final results into a tab delimited file. I then do the same again for a separate similar dataset ...
10
votes
1answer
1k views

How do you omit “./” when running scripts on current directory

On some tutorials over the internet they can run a script on the current directory without doing: ./script How do you omit this? I am using CentOS 5.6 x64. I want to run it this way script
10
votes
2answers
741 views

Is there a POSIX way of setting zeroth argument of a target application?

In bash you can use exec -a and in zsh you can alternatively also set ARGV0 to execute a program with a certain zeroth argument but is there also a POSIX way of doing so? As suggested in this one ...
10
votes
2answers
37k views

redirecting to /dev/null

I'm reading an example bash shell script: #!/bin/bash # This script makes a backup of my home directory. cd /home # This creates the archive tar cf /var/tmp/home_franky.tar franky > /dev/null 2&...