Command substitution is the replacement of a command with the result returned after it is evaluated.
Questions tagged [command-substitution]
234 questions
8
votes
3 answers
How to use `set -e` inside a bash command substitution?
I have a simple shell script with the following preamble:
#!/usr/bin/env bash
set -eu
set -o pipefail
I also have the following function:
foo() {
printf "Foo working... "
echo "Failed!"
false # point of interest #1
true # point of…
krispet krispet
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8
votes
1 answer
Multi-line, double quoted string triggers history expansion on subsequent single-quoted commands it gets piped to
I am on GNU bash, version 4.3.11.
Say I want to print unique lines on a file. I am using this approach, which works well on a file:
$ cat a
9
10
9
11
$ awk '!seen[$0]++' a
9
10
11
However, if I get the input from stdin, using double quotes in a…
fedorqui
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7
votes
2 answers
Capturing and testing output command in ZSH
I have tried countless ways to get what I want, but nothing seems to work. I always end up with something like 2:not found.
I want to capture the output of a command, and then test if it equals "!", like so:
function test() {
local…
user1218986
7
votes
1 answer
How to escape backticks in bash
I am trying to escape backtick in bash for long time.
I tried escaping with \ but it does not work.
Is it possible to escape backtick in bash?
Sample code
I="hello.pdf"
var1=`cat <
BhishanPoudel
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6
votes
5 answers
How to address error "bash: !d': event not found" in Bash command substitution
I am attempting to parse the output of a VNC server startup event and have run into a problem in parsing using sed in a command substitution. Specifically, the remote VNC server is started in a manner such as the…
d3pd
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6
votes
3 answers
Escaping backslash in AWK in command substituion
I am trying to escape backslash in AWK. This is a sample of what I am trying to do.
Say, I have a variable
$echo $a
hi
The following works
$echo $a | awk '{printf("\\\"%s\"",$1)'}
\"hi"
But, when I am trying to save the output of the same command…
Bill
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5
votes
2 answers
Can I use the $( ... ) syntax with multiline commands?
I often use backticks in with long commands that I break up with backslash-newlines for readability. When I replace the backticks with the $( ... ) syntax, I get errors.
For instance, I set a variable using curl and jq inside backquotes (reading…
August
- 343
- 3
- 10
5
votes
1 answer
Why does nesting quotes in backticks work in bash?
More precisely, why does
"`command "$variable"`"
treat the outer quotes as enclosing the inner quotes, instead of expanding the variable outside any quotes?
The exact command I used to test this is similar to an example brought up in another…
Larry
- 427
- 2
- 10
5
votes
2 answers
set environment variable in GDB from output of command
I am trying to exploit a buffer overflow in a challenge, the buffer gets it's value from an environment variable.
In GDB I know that you can set environment variables using the command:
set environment username = test
However I need to pass the…
imaibou
- 122
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5
votes
1 answer
Bash: Checking for exit status of multi-pipe command chain
I have a problem checking whether a certain command in a multi-pipe command chain did throw an error. Usually this is not hard to check but neither set -o pipefail nor checking ${PIPESTATUS[@]} works in my case. The setup is like this:
cmd="$snmpcmd…
user3040975
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- 10
4
votes
1 answer
POSIX shell: escaping line-continuations in backquote command-substitutions
I'm writing a shell, and I'm a little confused by the POSIX shell specification. Say I have the command:
echo "`echo "a\\
b"`"
Should the shell output
ab
or
a\
b
?
In other words, are line continuations removed again after removing the escaping…
programmerjake
- 1,794
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- 15
3
votes
1 answer
bash - commands in variables with pipes
Can someone explain why A and B behave differently?
A=`echo hello how are you | wc -w`
and
CMD="echo hello how are you | wc -w"
B=`$CMD`
They give different results:
$echo $A
4
$echo $B
hello how are you | wc -w
What I would like to have is a…
Martin Massera
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3
votes
1 answer
Bash scripting: Encoding string to base64 into a variable
I have a username and password variables. I want to encode them to base64 with the format of username:password.
#!/bin/bash
username=username
password=password
echo Encoding username/password...
echo $username:$password | base64
That works, but…
Gilbert Williams
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3
votes
2 answers
Trap with external call in command substitution breaks the parent Bash shell
I have a text-based user interface script that allows me to browse directories and select a file. The graphics are output to stderr, the chosen file's path is sent to stdout. This allows to get the chosen file this way:
file="$(./script)"
This is…
Informancien
- 255
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- 13
3
votes
0 answers
inlining command substitution in bash parameter expansion
How can I inline a command substitution in a bash parameter expansion instead of having to save the command substitution as a variable and then using that variable in the parameter expansion?
# inlined fails in bash; this works, however, in zsh
echo…
XDR
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