Questions about shell command substitution (such as $(command) or `command`), its mechanism, correct syntax etc. Not to be confused with aliasing.

learn more… | top users | synonyms

3
votes
2answers
100 views

When should one use $( ) in defining variables

Some shell scripts I have come across use the following syntax when defining variables: file_list_1=$(echo "list.txt") or file_list_2=$(find ./) I would have used: file_list_1="list.txt" and ...
2
votes
1answer
23 views

Return code for curl used in a command substitution

I've script as below :- httpUrl="http://www.nnin.com" rep=$(curl -v -X POST -d "UID=username&PWD=pass" $httpUrl) echo $rep when I run the script the return code always $?=0 because in a way ...
7
votes
1answer
130 views

Why there isn't a new line at the end of quoting a subshell and passing the results to echo?

When I do ls | grep png the output of grep is: 2015-05-15-200203_1920x1080_scrot.png 2015-05-16-025536_1920x1080_scrot.png (filename,newline,filename,newline) then, echo $(ls | grep png) outputs: ...
1
vote
2answers
36 views

Apply a filter to the value of a variable using backticks

Sorry for asking such a simple question, but reading about this issue on the internet has proven fruitless. I'm trying to better understand the grave/backtick operator and I've been doing okay so far ...
1
vote
1answer
45 views

How do I capture output into a variable and still have it goto the screen too?

I can do OUTPUT=$(grunt test) and that puts the output into OUTPUT but it's no longer on the screen. I need to see the output as it comes as well, so I can't just wait and then echo $OUTPUT. How can ...
2
votes
1answer
52 views

Bash - Command substitution adds single quotes

I have a problem with bash behavior while evaluating grave accents. I want to run find with optional -name: find -name "*.log" works fine, but when i want to get the -name part from nested script ...
-3
votes
1answer
248 views

does curl -Ls 'http://pastebin.com/raw.php?i=r2jYDZaw' mean it will not output anything?

Does the s in -Ls mean that nothing should be outputted? If so what will be the output then?
3
votes
1answer
71 views

how to send output of curl request to 2 separate commands

I'm piping the output of a curl request to gawk so that I can pull some data. The gawk code is already working if I'm searching through the html file which curl request produces. However, I was ...
2
votes
3answers
49 views

Command substitutions vs backslash escapes in a quoted string

Consider a double-quoted command substitution with backslash escapes inside it, like this: echo "$(echo '\\')" It prints out \\, whereas I would have expected it to print out only one backslash. My ...
0
votes
0answers
15 views

using man command for a system call close [duplicate]

So if we have an executable program called close on linux, and theres also a system call close. How can i generate a command that makes sure i get the manual for the system call "close" rather than ...
4
votes
3answers
44 views

Can command substitution be nested in variable substitution?

I would like to use variable substitution on a particular string that I access via a command. For example, if I copy something into my clipboard, I can access it like this. $ xclip -o -selection ...
2
votes
1answer
124 views

if command in find -exec

I was just trying to list all directories and files under current directory and also write if they are file or directory with the following command: find -exec echo `echo {} : ;if [ -f {} ]; then ...
0
votes
1answer
77 views

Bash escaping is confusing me

Lets say that I have a file (executable) called literally 'a\b'. It just echoes some stuff. Now I want to call that and I can do it by inputting user@machine:~/test$ ./a\\b into my console. Now ...
2
votes
1answer
256 views

Bash: command substitution with multiple unset/export commands

I am trying to unset some variables and export others using command substitution. This works: $(echo " export TEST_A=1 export TEST_B=2 ") And this too: $(echo " unset TEST_A ...
0
votes
2answers
34 views

Command substitution and spaces

Why does this work as a calculator: [root@qabun02 ~]# echo $(( 5+2 )) 7 but this doesn't? [root@qabun02 ~]# echo $( (5+2) ) -bash: 5+2: command not found The only difference I can see is the ...
14
votes
3answers
833 views

Any reason to use or teach `…` substition for new development?

It is my understanding that the more modern $(...) command substitution syntax is preferred over the old `-based syntax, due to easier and less error-prone nesting and escaping syntax. Further, it ...
2
votes
5answers
883 views

grep files from list

I am trying to run grep against a list of a few hundred files: $ head -n 3 <(cat files.txt) admin.php ajax/accept.php ajax/add_note.php However, even though I am grepping for a string that I ...
1
vote
1answer
432 views

Remote ssh command not sending echo variable from a ksh script on AIX

I'm need to get the value of a variable from a remote host in a script using SSH and have to source an environment file first that does a shift at the end of its run. The command works locally and I ...
4
votes
1answer
127 views

cmd2 `cmd1` vs cmd1 | xargs cmd2

I'm trying to find a file with find and then pass the found path to sqlite3. I'm only dropped into the sqlite> prompt if my command is of the form: sqlite3 `find . -type f -iname "*.db" | head ...
1
vote
2answers
202 views

Store stdout in variable and send command to background

I have a command in a bash script which I want to capture the output of and then send it to the background. How do I get this done? The following doesn't seem to work (it keeps blocking and outputs ...
3
votes
4answers
244 views

what is the difference between `find .` and /home/user/* as an input to for command

What is the difference between find . and /home/user/* as an input to for command.? For example: for var in $(find .) do echo "$var" done or for var in /home/user/* do echo "$var" done In the ...
3
votes
1answer
29 views

I need to create file with name is the result of the command in Solaris

I need to create only one file with name is the result of the command ls -l | head -2. I can't use environment variables. The filename must contain newline symbols. I've tried to use touch 'ls -l | ...
3
votes
5answers
979 views

Why do newline characters get lost when using command substitution?

I have a text file named links.txt which looks like this link1 link2 link3 I want to loop through this file line by line and perform an operation on every line. I know I can do this using while ...
0
votes
0answers
72 views

Why is a here-string command substitution considered as a single line?

I'm experimenting with while read syntax and there is a thing I don't understand. The most basic scenario which reproduces the problem is the following: a bash script a.sh calls another script b.sh ...
1
vote
2answers
72 views

How to perform command substitution before brace expansion

I would like to perform command substitution before brace expansion, but couldn't: $ ls {$(seq -s , 13 20)}.pdf ls: cannot access {13,14,15,16,17,18,19,20}.pdf: No such file or directory How can I? ...
0
votes
2answers
199 views

Bash Mysql Command Substitution Login Failure

Bash and MySQL versions: GNU bash, version 4.2.25(1)-release (x86_64-pc-linux-gnu) mysql Ver 14.14 Distrib 5.5.37, for debian-linux-gnu (x86_64) using readline 6.2 I have a bash script that tries ...
2
votes
2answers
56 views

Is there an improved version of “pick” that will work with filenames with spaces?

Is there a version of the pick script that will work properly with spaces in the filenames? This is a tricky question—pick is trivial to enhance to work with spaces in filenames, but the output ...
2
votes
2answers
92 views

Reference a path name in `find` output results

What I would like to do is to be able to reference a find path name result as a parameter to another command without laboriously typing the full path name. Example: $ find -name testfile path1 ...
3
votes
1answer
98 views

How do I diff the outputs of two commands?

How can I use the diff command to compare 2 commands' outputs? Does something like this exist? diff ($cat /etc/passwd) ($cut -f2/etc/passwd)
2
votes
1answer
175 views

Why doesn't PIPESTATUS work on pipelines within a command substitution

Proof below: Bash 4 Prompt> $(echo hello|sed 's/h/m/'|xargs -I{} ls {} 2>/dev/null|sed 's/ /_/') Prompt> for i in ${PIPESTATUS[@]}; do echo $i;done Output> 0 Prompt> echo hello|sed ...
1
vote
2answers
114 views

Why asterisk in command result in for loop is expanding?

I want to write a script that will run git diff --name-status master..<BRANCH> but when I run this: for i in $(git branch | grep -v master); do echo $i; done I get echo one directory ...
1
vote
1answer
51 views

S-Lang shell (slsh): capture command output in a variable

In slsh I know that I have to !-prefix a command to execute it: !ls How can I capture its output in a variable?
3
votes
2answers
182 views

Command substitution declaration syntax in bash - which of these two is better practice? [duplicate]

This is part of a bash find loop, and I wondered which is more correct syntax and why? filename="$(echo "$i" | cut -c5-)"; filename=`echo "$i" | cut -c5-`; Both function for the purpose of getting ...
5
votes
2answers
140 views

“command | less” vs “less <(command)”

A common "idiom" for viewing large amounts of command output is to pipe it to less, via command | less. However, it's also possible (perhaps only in bash, haven't tested in other shells) to use the ...
3
votes
2answers
218 views

How can I list the info for an array of /dev/disks using bash expansion or substitution?

I want the output of: diskutil list; diskutil info [multiple devices] Without having to do: diskutil info disk0; diskutil info disk0s1; diskutil info disk1 ...etc For example, with many builtin ...
1
vote
1answer
40 views

Does the command execute automatically when setting it to a variable?

I'm getting familiar with all about linux & shell/bash scrpiting. I'm configuring a backup script & it will compress an entire dir. I wanna know if I have to print the variable where the ...
4
votes
3answers
399 views

How do I read the output of commands in a script

I'm learning Linux scripting at the moment, and one thing I can get working is assigning a command output to a variable. The command service httpd configtest returns Syntax is OK, so I wrote this. ...
5
votes
2answers
142 views

Command substitution vs I/O Redirection?

I am wondered about differences between I/O Redirection and Command substitution? For example when I want to redirect which command output to cd command (I know that it's not a directory) then the ...
0
votes
1answer
283 views

Commands inside bash not being interpreted?

I'm trying to execute a command inside a bash script, $(pwd) is not being interpreted at all, and not even using eval. This do not work: cat apache-vhost.conf | sed 's/{path}/$(pwd)/g' Neither ...
2
votes
2answers
250 views

grep and directories containing spaces

I would like to search all files containing an association of strings (here the files should contain 'arthur' AND ('déni' OR 'deni') for instance) in a directory and subdirectories. After some search ...
22
votes
2answers
7k views

Quoting within $(command substitution) in Bash

In my Bash environment I use variables containing spaces, and I use these variables within command substitution. Unfortunately I cannot find the answer on SE. What is the correct way to quote my ...
1
vote
2answers
56 views

subsitute removing zero numbering

I would like to subsitute in a txt file the followings targets for example "code_000040" by "code_40" and "code_067340" by "code_67340" and so on.., in other words to remove the zeros in the 6 ...
3
votes
2answers
152 views

Inconsistencies between redirecting input from file, here docs, and here strings

Why is this inconsistent? I would expect here docs and here strings to be functionally equivalent to redirecting input from a file. $ bash --version GNU bash, version 4.1.2(1)-release ...
2
votes
1answer
81 views

Assignment to variables

I was trying to devise a solution to a different question asked on the site and came across this curious issue. Here is the set of commands that I issued: $ foo=82a9948422 $ echo $foo 82a9948422 $ ...
3
votes
1answer
645 views

variable expansion within command substitution over SSH bash 4.X

I want to do this ssh ${w100user}@web100 'ls -l "$(grep "${1}" /etc/pure-ftpd/pureftpd.passwd|cut -d':' -f6)"' Which obviously performs an ssh session to server web100 as w100user and then greps ...
1
vote
1answer
2k views

Basename gives extra operand error

I 'm using the following script to merge few pcap files into one using mergecap command. But when I run it it gives me a 'basename: extra operand /mnt/md0/capture/DCN/dcn_2014_02_04_00_11_47_598.pcap' ...
5
votes
5answers
894 views

Using a generated list of filenames as argument list — with spaces

I'm trying to invoke a script with a list of filenames collected by find. Nothing special, just someting like this: $ myscript `find . -name something.txt` The problem is that some of the pathnames ...
1
vote
1answer
570 views

How to escape spaces while using command's output in command line [duplicate]

I want to perform the following command in command line : $ md5sum $(find . -type f) But this would cause problems when it encounter files with spaces in filenames : md5sum: Kaufmann: No such ...
3
votes
2answers
261 views

Command substitution interpreting spaces within quotes as argument separators

I'd like an alias that additionally appends itself to ~/.bashrc, e.g. function tailias { $(echo "alias $1='${*:2}'" | tee -a ~/.bashrc) } I'm using tee to split the command to ~/.bashrc while ...
5
votes
5answers
20k views

Pass the output of previous command to next as an argument

I've a command that outputs data to stdout (command1 -p=aaa -v=bbb -i=4). The output line can have the following value: rate (10%) - name: value - 10Kbps I want to grep that output in order to ...