Command substitution is the replacement of a command with the result returned after it is evaluated.
Questions tagged [command-substitution]
234 questions
1
vote
1 answer
bash: Run a command once and save stdout and stderr in their own variables
I need to ensure a command is outputting the correct text. I currently have:
command_stdout="$(mycommand --flag 2>/dev/null)"
command_stderr="$(mycommand --flag 2>&1 1>/dev/null)"
Instead of having to run the same command twice, is there any way I…
leetbacoon
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1
vote
4 answers
Execute command that results from execution of a script whose name is in a variable
When posting this question originally, I totally misworded it, obtaining another, reasonable but different question, which was correctly answered here.
The following is the correct version of the question I originally wanted to ask.
In one of my…
Enlico
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1
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3 answers
Run command in script and check return value
I am trying to run the diff command on two folders, check the return value and then output a message.
I have this:
#!/bin/bash
result='diff dir1 dir2'
if result == 0
then
echo "OK"
else
echo "ERROR"
fi
But I am getting result: command…
user997112
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1
vote
1 answer
Run perl script in bash array
I need to run a perl script in a bash array that will return certain values based on the string you provide. The perl script itself takes a user and a string and it will return the value based on the string you give it. For example. This is how the…
Michael
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1
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0 answers
Why is '$(< file)' equivalent to '$(cat file)' in bash?
According to bash manual:
The command substitution $(cat file) can be replaced by the equivalent but faster $(< file).
but I fail to see why. Let's try to take a closer look at what $(< file) means and what it does:
by definition, a $(command)…
Peter
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1
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2 answers
How to remove a known last part from commands output string in one line?
To rephrase - I want to use Bash command substitution and string substitution in the same line.
My actual commands are longer, but the ridiculous use of echo here is just a "substitution" for shortness and acts the same - with same errors ;)
I know…
uldics
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1
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1 answer
Why isn't a semicolon in command substitution output treated identical to one in the original code?
In my understanding of command substitution this should work, but it doesn't, can you explain me why and how to do something like this.
Why does this work:
cd ..; echo 123 # output is "123", after changing directories
...when this…
Salmjak
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1
vote
1 answer
Unable to echo last executed within .sh script
Using following on
cmd=$(uname -a)
printf "["!:0"]"
gives me
[cmd1=Linux localhost.localdomain 2.6.32-300.10.1.el5uek #1 SMP Wed Feb 22 17:37:40 EST 2012 x86_64 x86_64 x86_64 GNU/Linux]
but when is done in test.sh
#!/bin/bash
output=$(uname…
lazereyes
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1
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1 answer
How do I include a variable in a ksh command subtitution?
I'm trying to find a number of lines that match a regex pattern in grep received as a variable. When I do the grep with the pattern directly in the command substitution, it works. When I use a variable for the pattern, it…
Pat N.
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1
vote
2 answers
Parent trap visible but not run by subshell
Tested for Bash 5.0.2
According to the GNU Bash Reference Manual,
Bash performs the expansion [of a command substitution] by executing [the] command in a subshell environment
According to The Open Group Base Specifications Issue 6:
when a…
Marcus Rossel
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1
vote
1 answer
Replace a date string in sed with the ouput of a custom function with arguments
I have a bash script which echos out an html file like
this ./foo.sh > out.html In the html there are timestamps in the
following format 2019-02-24T17:02:19Z.
I wrote a function to
convert the time stamp to the time delta between the timestamp
and…
user3022779
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1
vote
1 answer
Use substituted string as a command in shell script
Given the variables listed below:
echo ${userupper}_PYTHON
# YTU_PYTHON
echo $YTU_PYTHON
# /home/ytu/anaconda3/bin/python
echo $path
# foo.py
Now I'd like to execute /home/ytu/anaconda3/bin/python foo.py with userupper and path. I tried…
ytu
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1
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2 answers
csh: set: No match
I define a function, an array and a variable:
set fnctn = "F(x)=Vx1*(1+cos(1*x-pi))"
set Vx = ( 1 1 1 1 )
set Vx1 = $Vx[1]
The following commands do what I want:
echo "$fnctn" | sed "s/Vx1/$Vx1/"
set fnctn2 = `echo "$fnctn" | sed…
fyd
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1
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4 answers
IFS and command substitution
I am writing a shell script to read input csv files and run a java program accordingly.
#!/usr/bin/ksh
CSV_FILE=${1}
myScript="/usr/bin/java -version"
while read row
do
$myScript
IFS=$"|"
for column in $row
do
$myScript
…
Mayavi
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1
vote
1 answer
Bash script - perform grep on filenames containing particular numbers
I have a bunch of files such as:
jobReports/NARX_20191212_1.out
jobReports/NARX_20191212_10.out
jobReports/NARX_20197695_2.out
jobReports/NARX_20197695_3.out
jobReports/NARX_20261798_1.out
where the first numeral sequence denotes a jobID, and the…
Asy
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