25
votes
Accepted
How do bash loops work precisely?
Ordinary for loops always iterate over a static set of strings. This is regardless of whether the strings are generated by brace expansions or by filename globbing patterns, or some other expansion of ...
- 356k
23
votes
Accepted
In for loops in bash, is the counter variable local or global?
for doesn’t introduce its own variable scope, so i is whatever it is on entry to the for loop. This could be global, or local to whatever function declared it as local, or even global but in a sub-...
- 482k
22
votes
Accepted
How to loop for 3 times in bash script when docker push fails?
Use for-loop and && break:
for n in {1..3}; do
docker push $CONTAINER_IMAGE:latest && break;
done
break quits the loop, but only runs when docker push succeeded. If docker push ...
- 23.2k
17
votes
Accepted
Looping over dirs using `find . -depth 1 -type d`
The -depth switch does not take an argument, but -maxdepth does, so:
for dir in `find . -depth -maxdepth 1 -type d`
do
....
should work.
The -depth argument as per the man page means process ...
- 2,751
13
votes
Accepted
Adding suffix to filename during for loop in bash
for file in TLC*.csv; do
cut -d, -f2- "${file}" > "${file%.*}_prepped.csv"
done
${file%.*} removes everything after the last dot of $file. If you want to remove everything ...
- 19.4k
13
votes
Bash for loop with string var containing spaces
If you're using bash you can use an array for this
#!/bin/bash
files=('foo bar' 'another file' file1 'file2')
for f in "${files[@]}"; do file -- "$f"; done
Quoting is required for ...
- 128k
13
votes
Accepted
How can I assign the output of a command to different variables in each loop iteration?
In your case, I would use an associative array for that:
declare -A rules
for chain in http https ssh
do
rules[$chain]=$(iptables -nvxL $chain | tail -1 | awk '{print $2}')
done
You can then ...
- 23.6k
12
votes
echo list / array to xargs
printf '%s\n' "${list[@]}" | xargs
This would print each element of list on its own line and that newline-delimited list would be passed to xargs.
"${list[@]}" would expand to the ...
- 356k
12
votes
Accepted
changing IFS temporarily before a for loop
for is a reserved word and as such follows special rules:
The following words shall be recognized as reserved words:
! { } case do done elif else esac fi for if in then until while
This recognition ...
- 19.4k
11
votes
Accepted
How to convert an input parameter to integer value in a for loop in bash?
You can do this two ways:
With ksh93-compatible shells (ksh93, zsh, bash):
for (( i=1;i<=$2;i++ ))
do
echo "Welcome $i times"
done
Here we set i to 1 and loop, incrementing it until it ...
- 1,442
11
votes
bash loop to replace middle of string after a certain character
I think you were very close to a working command. This worked for me on the few examples you gave:
sed -E 's/_[0-9]+ /|/' "$file" > "$file.1"
I changed the match expression ...
- 7,221
10
votes
for loop not working in bash
The $(foo) construct will run the command foo and replace $(foo) with the output of running foo. You want a glob, that's not a command. What you're doing is attempting to run all files called ./zebu....
- 252k
10
votes
Accepted
How to run a loop inside sh -c
The usual wisdom is to define the script (after the -c) inside single quotes. The other part you need to use is a shell where the {1..4} construct is valid:
$ bash -c 'for i in {1..4}; do echo $i; ...
- 2,123
9
votes
Accepted
Why does bash replaces text from command substitution with text thereafter
The problem is that your files have DOS/Windows-style line-endings. As a quick work-around, replace:
echo "$(cat "$enabled") |"
With:
echo "$(tr -d '\r' <"$enabled") |"
Here, tr removes the ...
- 76.4k
9
votes
Accepted
Weird bash behavior when IFS is set for a for loop
Keywords aren't recognized after an assignment. So, the for in IFS=blah for ... just runs a regular command called for, if you have one:
$ cat > ./for
#!/bin/sh
echo script for
$ chmod +x ./for
$...
- 148k
9
votes
loop through one file extension or the other in bash or zsh
You don't. You iterate over both "extensions" and (if you need to) check if either or both exist. e.g. if you only want to print one or the other (the first one seen), keep a list of the ...
- 84.4k
8
votes
Accepted
Bash: Multiple for loops in Background
The outer loop that you have is basically
for i in {1..10}; do
some_compound_command &
done
This would start ten concurrent instances of some_compound_command in the background. They will be ...
- 356k
8
votes
Accepted
Bash script - variables in curly braces
Variables won't expand inside brace expansion. You could do:
for ((number=startNumber; number<=endNumber; number++)); do
echo "$number"
done
Also, there is no reason to use ...
- 41.6k
8
votes
Bash script - variables in curly braces
As explained elsewhere the expansion won't work. Alternative way to get your sequence of numbers:
for number in $(seq $startNumber $endNumber)
do
echo $number
done
- 5,210
8
votes
Accepted
Loop through MAC addresses, how to handle both numbers (0-9) and letters (a-f) in a "for" loop
Just add another brace expansion for the letters:
for i in {2..9} {a..f}
do
grep "Node${i}\|01, source address = 00:00:00:00:00:0${i}" t1.txt > t2.txt
done
Note that this is likely ...
- 252k
7
votes
Accepted
Rename files using a WHILE loop instead of a FOR loop
There's nothing wrong with using a while loop here. You just have to do it right:
set -- *.jpeg
while (($#)); do
mv -- "${1}" "${1/DSC/Paris}"
shift
done
The while loop above is just as reliable as ...
- 85.7k
7
votes
Loop to create subdirectories in multiple directories
Assuming the m3* directories exist,
for dir in m3z m3t m3t2 m3g m3g2 m3g3; do
mkdir "$dir"/{log,lib,txt}
done
or, with brace expansion on the list of directories,
mkdir {m3z,m3t,m3t2,m3g,m3g2,...
- 356k
7
votes
Accepted
Inner for loop when run in background in bash spawns new bash process
Because you have piped the loop into grep, it must be run in a subshell. This is mentioned in the Bash manual:
Each command in a pipeline is executed in its own subshell, which is a separate process ...
- 78.9k
7
votes
Run script for 10 times or until it meets the condition
The break builtin is used for this.
for i in {1..10}; do
sleep 10
OUTPUT=$(systemctl is-active etcd)
if [[ $OUTPUT == active ]]; then
echo "The result is successful"
break
...
- 41.6k
7
votes
Accepted
How to watch a for-loop in bash?
watch's command argument(s) are a script that is run with sh -c.
If the command arguments are just a list of tokens separated by spaces (e.g. watch ls -l), it concatenates them all and runs them. ...
- 84.4k
7
votes
For loop through a variable vector
In Bash and ksh, referencing an array without an index ($foo) is the same as referencing it with an index of 0 (${foo[0]}). The way to get all elements of the array is to index with the special value @...
- 148k
6
votes
Accepted
Using a For Loop to Sort and Save Files Using Grep
(in adition to @Philippos): Bash is trying to expand variable $i_ instead of $i.
Try ...${i}_...:
for i in {00..21}
do
grep "text" file_${i}_*out > out_$i.txt
done
- 12.8k
6
votes
how can we use multiple variables in single for loop in shell script?
$ cat inputfile.txt
foo|bar
baz|foobar
$ cat inputfile.txt | while IFS='|' read i j ; do echo $i:$j ; done
foo:bar
baz:foobar
this answer is a lot like this answer but it also answers the comment ...
6
votes
How do I get 0-padded numbers in {} (brace expansion)?
Although Brace expansion has a zero-padding method, I found Parameter expansion more useful to alter existing variable:
digits=00000
num=123
# Option 1:
temp=$digits$num
echo ${temp:(-${#digits})}
# ...
- 1,129
6
votes
Force alphabetical order in for loop with if conditions
#/bin/sh
# shellcheck disable=SC2046
# ^ word-splitting by the shell is intentional in this file
elems="Cr Hf Mo Nb Ta Ti V W Zr"
for a in $elems
do
for b in $elems
do
for c in $...
- 8,418
Only top scored, non community-wiki answers of a minimum length are eligible