Questions tagged [command-substitution]
Questions about shell command substitution (such as $(command) or `command`), its mechanism, correct syntax etc. Not to be confused with aliasing.
388
questions
0
votes
1
answer
70
views
When simply `| cat -`, I get file descriptor errors
Here's the working diff command:
$ diff -u <(echo 'foo:bar:baz' | tr : "\n") <(echo 'foo:baz' | tr : "\n")
--- /dev/fd/11 2023-08-30 13:11:50
+++ /dev/fd/13 2023-08-30 13:...
- 101
2
votes
2
answers
77
views
Parse command with expanded variables
Trying to test something and not quite nailing it either way I've tried, so not sure if I'm being silly or not.
Essentially I need to parse a string to ipa-getcert's post-save argument which should ...
- 21
2
votes
2
answers
3k
views
Bash variable substitution into yq command
Given a YAML file, example.yaml:
node:
sub_node:
get_this:
I'd like to get a variable containing get_this using Mike Farah's yq and the string sub_node
yaml="$(cat example.yaml)"
...
- 491
0
votes
2
answers
262
views
Command substitution not working with environment variable
I have this command which I want to variabilize. Particularly I want the latest argument presence to depend on an environment variable.
ansible-playbook --inventory inventories/ssg-dev deploy.yml --...
- 103
0
votes
0
answers
33
views
process substitution vs command substitution
I am trying to use vcf-merge command. The command works like this:
vcf-merge f1.vcf.gz f2.vcf.gz f3.vcf.gz
I have bunch of vcf.gz files so I did something like this:
vcf-merge <(ls *.vcf.gz | tr &...
- 2,441
0
votes
0
answers
33
views
What does !^ (exclamation mark caret) do in bash? [duplicate]
Does anyone know what the command !^ does in the bash shell? I know ! relates to history expansion and ^ relates to command substitution (when it's not being used in a regular expression when it ...
- 123
0
votes
1
answer
108
views
Loop to Monitor Line Count Growth Using wc -l
I've written a short bash script to monitor the line count growth (wc -l) in a file which is receiving results from a loop.
Thus:
printf "Name of file to monitor\n"
read file
printf "...
- 17
0
votes
1
answer
60
views
How to pipe output of a command to another command (which does not take a file as input?) [duplicate]
What I want
Simplified command: echo "helloworld" | echo $1
I know, this would be possible with cat, but what if a command like echo does not take a file as input?
My actual command: ls -...
- 540
1
vote
0
answers
50
views
How should alias in command substitution behave?
I tested a few scripts with different shells, and the results differ.
# test 1
foo(){ echo $(al);}
alias al='echo 123'
foo
# test 2
alias al='echo 123'
foo(){ echo $(al);}
alias al='echo abc'
foo
# ...
- 11
0
votes
2
answers
36
views
How to atribute multiple values to an array from a command substitution?
Consider the following shell script function:
#!/bin/bash
declare -a dir
function() {
local -a directories=( "A/B/C D" "E/F G H" ) #Initialize local array.
printf "%q " ...
- 1
0
votes
0
answers
33
views
Distribute subcommands over head command to avoid writing same words twice?
I would like to try to write this command:
while read -r repo; do gh repo delete $repo; done <<< $(gh repo list --no-archived)
which basically cleans up my GitHub and deletes everything that ...
- 143
0
votes
0
answers
54
views
Bash command substitution in function with redirect
I am trying to pass a command that contains I/O redirection to a function. The function executes the command using command substitution and redirects stderr to stdout. The command should return an AWS ...
- 1
0
votes
0
answers
122
views
recursive history search for tcsh shell
In bash shell, i generally use CTRL+R to fuzzy search a command that i executed recently.
However the same doesn't seem to work for tcsh or csh shells.
I want to understand if/how to do the same ...
0
votes
1
answer
218
views
referencing a ${variable} value inside a sed substitution
I am trying to replace a string within a couple of similar sql scripts.
in my toolbox I have a for loop as well as grep and sed inside a bash script.
#!/usr/bin/env bash
for efa_instance in ...
- 139
0
votes
3
answers
34
views
Iterate over a multiline variable and extract a particular line and use it as a command
The variable I wish to extract will look something like this
ttyACM0
ttyACM1
I would like to iterate over this variable and issue the command:
echo disconnect > /dev/ttyACM0
and then
echo ...
- 1
0
votes
1
answer
100
views
Syntax Question for Bash using pipes
I am trying to set a variable to a value returned by a program (lynx). I cannot get past which parenthesis or bracket I use to accomplish my goal.
I have this:
DEBFILE=${(lynx -listonly -dump https://...
0
votes
1
answer
81
views
How to take the number including dot from an output of one command and use it in another one?
How to take the number including dot from an output of one command and use it in another one?
For example, this command
chia wallet show -w standard_wallet
gives this result
Wallet height: xxxxxx
...
- 23
3
votes
2
answers
446
views
Parameter substitution inside indirect expansion
I want to achieve the parameter substitution inside indirect expansion, but I cannot achieve it with a single command.
I get a "bad substitution" error. Let me know if there's a clever way ...
1
vote
1
answer
27
views
Can bash_history save evaluated commands output?
Is there a way for bash or other shells' history to save the output of evaluated commands when it's a parameter?
$ echo $(pwd) && history
/root
0 I want something like:
1 echo $(pwd) &...
- 113
-1
votes
1
answer
148
views
Call command with one argument being the result of cat'ing a file
How do I call a command with on argument being the result of cat'ing a file?
npx aws-api-gateway-cli-test \
--username $username \
--password $password \
--method $method \
--body cat user....
-1
votes
2
answers
68
views
How to use the string returned by $() to be used as more than one argument in another command?
My use case:
I need to remove all dev packages listed in composer.json file. Suppose I have two packages: projectx/package-nice and projecty/package-good. To remove them I need to run:
$ composer ...
- 329
2
votes
2
answers
668
views
Echoing/printing a bash `ls` command substitution without quotes [duplicate]
I have the following example:
$ a="$(ls)"
$ echo $a
backups cache crash lib local lock log mail opt run snap spool tmp
$
$ echo "$a"
backups
cache
crash
lib
local
lock
log
mail
...
- 293
0
votes
2
answers
739
views
fish shell: why does `file `which command`` work in zsh and bash, but not fish?
Before switching to fish shell, I frequently used various commands in zsh with which some_command. An example might be:
$ file `which zsh`
/opt/local/bin/zsh: Mach-O 64-bit executable arm64
/bin/zsh: ...
- 611
0
votes
2
answers
163
views
Is it possible to generate multiple complete commands with command substitution?
There are many commands that accept multiple parameters, like touch, ls and rm. So that I can use the construct {1..3} to generate multiple parameters for the command. For example:
[root@192 ~]# touch ...
- 965
1
vote
2
answers
399
views
How to execute a string of bash command with command substitution
Suppose I have a bash variable like this:
tmp1='$(echo foo)'
or
tmp2='`echo foo`'
How to achieve foo as result?
I know that removing the command substitution from the string should work, but is ...
- 111
0
votes
1
answer
58
views
Why does $(RANDOM) work differently from $(ls -la)?
for command substitution in bash, we do this
hello=$(ls -la)
echo "$hello"
but when i do the same for below command,it fails,it says RANDOM not found
hell=$(RANDOM)
echo "$hell"...
- 115
3
votes
1
answer
109
views
Don't run command when command substitution result is blank
I have the following code to run pylint on all files in a Git repository with file extension .py, fed by command substitution.
pylint $(git ls-files '*.py')
The script runs just fine except when ...
- 133
0
votes
0
answers
22
views
Handling whitespaces in the parameters into a command substitution [duplicate]
I have troubles with whitespaces and command substitution. I made a tiny script to show the problem:
#! /bin/bash
a="toilet -t --font='Dot Matrix' --filter metal --filter border ABC"
#a=&...
- 234
0
votes
1
answer
370
views
Command substitution with xargs inputs [duplicate]
I'm trying to write a simple command that will create symlinks for all files in a specific directory (Yes, exactly like lndir but I cannot use it so I try to mimic it).
I tried the following using ...
- 143
2
votes
1
answer
189
views
What's the differences? (command substitutions) [duplicate]
printf "%s" `echo ../.. | sed 's/[.]/\\&/g'`
printf "%s" $(echo ../.. | sed 's/[.]/\\&/g')
printf "%s" "$(echo ../.. | sed 's/[.]/\\&/g')"
- 21
1
vote
2
answers
195
views
Can someone tell me the difference between these three command substitutions?
I am asking why these three commands give three different answers:
$ printf "%s\n" `echo ../.. | sed 's/[.]/\\&/g'`
&&/&&
$ printf "%s\n" $(echo ../.. | sed '...
- 13
5
votes
3
answers
4k
views
passwordless check to see if a user can use 'sudo'
I want to check if a user is able to use sudo - but without the need to write the password - for bash shell scripts purposes. I did some research and I found:
Sudo - is there a command to check if I ...
- 1,456
0
votes
2
answers
842
views
How to use string substitution in one command with command substitution?
When the git branch command is executed it usually shows:
* main
.... # other branches
Having the following through command substitution and string substitution work as expected:
current_branch=$(...
- 1,456
0
votes
3
answers
159
views
How to retrieve the command executed by command substitution for log reports
For Simplicity, consider the following:
x=$(java --version)
echo "x: $x"
It works how expected showing:
x: openjdk version "11.0.13" 2021-10-19
OpenJDK Runtime Environment Temurin-...
- 1,456
0
votes
0
answers
28
views
Behaviour for output of Command Substitution working with String variable with Pipes [duplicate]
For simplicity and how source/input of this post for the coming scenarios, observe the following
java --version
openjdk 11.0.13 2021-10-19
OpenJDK Runtime Environment Temurin-11.0.13+8 (build 11.0.13+...
- 1,456
0
votes
1
answer
234
views
mount cifs: how to retrieve the status of the command execution from the terminal?
The following mount cifs code works fine:
sudo mount -t cifs //192.168.1.77/something /mnt/192.168.1.77/something \
-o username=alpha,rw,uid=alpha,gid=tango,dir_mode=0770,file_mode=0770,...
- 1,456
0
votes
2
answers
881
views
Bash: Alias not recognized in shell interpolation
~$ bash --version
GNU bash, version 5.1.12(1)-release (x86_64-pc-linux-gnu)
~$ alias bab=python
~$ $(echo bab)
bash: bab: command not found
I'd expect bab to be turned to "python", but it ...
- 143
0
votes
1
answer
105
views
Incorrect order of commands in "Command Substitution" (Bash/Dash)
Why is it that when you run the substitution commands, the order of execution is broken, as in the example below. First the chmod command was executed, then echo 1 and echo 2?
echo $(echo 1; echo 2; ...
- 147
0
votes
1
answer
105
views
Wrong arguments being passed when sourcing script using command substitution [duplicate]
I have a script print.sh:
#!/bin/bash
echo printing provided args:
for i in "$@"; do
echo -e "\t${i}"
done
If I do this at the prompt a=$(. print.sh ); echo "${a}&...
- 77
2
votes
1
answer
96
views
Backticks giving different result than pasting result
GNU bash, version 5.0.17(1)-release (x86_64-pc-linux-gnu)
Ubuntu 20.04.3 LTS
So, I downloaded like 20 7z files and I wanted to move them somewhere, but NOT the previous 7z files I already had in ...
- 229
3
votes
2
answers
10k
views
How do I ignore "command substitution: ignored null byte in input"?
I have this Linux shell command:
echo $(python3 -c 'print("Test"+"\0"+"M"*18)') | nc -u [IP] [PORT]
My intention is to pipe the output of the print statement to the ...
- 169
2
votes
1
answer
365
views
Bash: command substitution with background job
I'm trying to fire and forget a process within a function in bash.
I'd this function to return a status corresponding to the fact the process has indeed been fired and retrieve output of the function ...
- 123
-2
votes
1
answer
681
views
How to run a command using another command output as part of its text in terminal?
If command1 is:
curl -k -v -u user:password https://example.com/v2/image/manifests/tag -H Accept: application/vnd.docker.distribution.manifest.v2+json 2>&1 | grep Docker-Content-Digest | awk '{...
- 269
8
votes
1
answer
4k
views
Makefile command substitution
My Makefile:
all: ...(other rules) clean
clean:
rm $(find . -type f -executable)
When I delete clean rule from the above Makefile everything works as expected. After adding, make (also make ...
- 181
1
vote
1
answer
3k
views
"bash: syntax error near unexpected token `('" error with process substitution
In a fresh, clean bash instance I get as expected the following:
$ printf "a\td\n" | cat -vE
a d$
$
$ file <(printf "a\td\n" | cat -vE)
/dev/fd/63: symbolic link to pipe:[...
- 260
1
vote
1
answer
2k
views
Environment variable expansion inside $(command substitution)
I'm running Bash 5.1.4 on Debian.
I'm writing a post-installation script to copy configuration and other files to locations in my home directory. I add the intended destination to each file at the ...
- 337
3
votes
2
answers
209
views
Why does the command echo `echo \\\\\\\z` in bash script print \\z instead of \\\z?
The command
echo `echo \\\\\\\z`
is from this book , I don’t understand why it prints
\\z
when it get executed via the bash script.
I think it should print
\\\z
- 31
4
votes
3
answers
2k
views
How to have Bash interpret output from command as quoted strings?
I have a program that gets the files selected in the graphical IU (in my case Finder in macOS). The output is something like
'/tmp/file number one.txt' '/tmp/file number two.txt'
Note the space char ...
- 41
1
vote
1
answer
58
views
Bash: Difference between paste command with Command Substiution vs. FIFO FDs
I'm just trying to understand why when I use a couple FIFO FDs in a paste command I get a 'Broken Pipe' but when I use Command Substitution everything is all good.
A little background: I have a need ...
- 21
0
votes
1
answer
340
views
Saving hashed string in a variable
I need to hash a string with SHA-512 algoithm. I have a salt also.
I'm trying in the script this:
inputHash=$( perl -e 'print crypt("${array[3]}","\${pass[0]}\$${pass[1]}\$") . &...
- 1