62
votes
Accepted
Why won't the strings command stop?
If GNU cat can't write out what it read, it will exit with an error:
/* Write this block out. */
{
/* The following is ok, since we know that 0 < n_read. */
size_t n = n_read;
if (...
- 4,729
55
votes
Accepted
What is the application of `rev` in real life?
The non-standard rev utility is useful in situations where it's easier to express or do an operation from one direction of a string, but it's the reverse of what you have.
For example, to get the last ...
- 356k
35
votes
Accepted
How to split a string by the third .(dot) delimiter
Used sed for example:
$ echo 'version: 1.8.0.110' | sed 's/\./-/3'
version: 1.8.0-110
Explanation:
sed s/search/replace/x searches for a string and replaces it with another string. x determines which ...
- 9,276
31
votes
Accepted
Using the diff command to compare two strings?
Yes, you can use diff on two strings, if you make files from them, because diff will only ever compare files.
A shortcut way to do that is using process substitutions in a shell that supports these:
...
- 356k
26
votes
Accepted
In `sed` how can I put one "&" between characters in a string?
With GNU sed:
sed 's/./\&&/2g'
(substitute every (g) character (.) with the same (&) preceded with & (\&) but only starting from the second occurrence (2)).
Portably:
sed 's/./\&...
- 585k
25
votes
How to generate a random string?
Use the xxd command to specify length (via -l),
which works both in Linux and macOS.
See the xxd(1) man page
or the Linux xxd Command Tutorial for Beginners (with Examples).
xxd -l16 -ps /dev/urandom
- 359
24
votes
Accepted
How to split a string by ':' character in bash/zsh?
The : there is any arbitrary character:
You can use:
parts=(${(s/:/)str})
Some common character pairs are also supported like:
parts=(${(s[:])str})
If you're going to use the @ flag to preserve ...
- 585k
23
votes
Padding trailing whitespaces in a string with another character
filler='===================='
string='foo'
printf '%s\n' "$string${filler:${#string}}"
Gives
foo=================
${#string} is the length of the value $string, and ${filler:${#string}} is the ...
- 356k
22
votes
Accepted
How to split a string in two substrings of same length using bash?
Using parameter expansion and shell arithmetic:
The first half of the variable will be:
${var:0:${#var}/2}
The second half of the variable will be:
${var:${#var}/2}
so you could use:
printf '%s\n' ...
- 41.5k
21
votes
Accepted
What's the purpose of adding a prefix on both sides of a shell variable comparison to a string literal?
The important thing to understand here is that in most shells¹, [ is just an ordinary command parsed by the shell like any other ordinary command.
Then the shell invokes that [ (aka test) command with ...
- 585k
19
votes
Accepted
Split string on colon in /bin/sh
Just do:
case $1 in
(*:*) host=${1%:*} port=${1##*:};;
(*) host=$1 port=$default_port;;
esac
You may want to change the case $1 to case ${1##*[]]} to account for values of $1 like [::1] (...
- 585k
18
votes
Accepted
How to grep '$$$$$'?
It seems that $ is a special character for end of line?
Yep, exactly. And there's an end-of-line on each and every line.
You'll need to use \$\$\$\$\$ as the pattern, or use grep -F '$$$$$', to tell ...
- 148k
16
votes
Padding trailing whitespaces in a string with another character
printf "%.20s:\n\n" "$str========================="
where %.20s is the string truncating format
- 12.8k
16
votes
How to compare 2 strings in UNIX shell script?
Simple answer to do so you can use this syntax :
if [ "$category" = "$request" ]
Spacing is important.
Side note, you should use the recent way of doing command subsitition and replace (see article 1 ...
- 9,882
16
votes
Accepted
Build a command dynamically
Don't try to make an executable string. Instead build the arguments in an array and use that when calling tar (you are already using an array properly for EXCLUDE):
#!/bin/bash
directory=/tmp
...
- 356k
16
votes
Accepted
Pattern matching in a zsh conditional expression
[ $x = '*test*' ] tests whether the string resulting from expanding $x, which is text, is equal to the string resulting from expanding '*test*', which is *text*.
To test whether the value of the ...
15
votes
How to use the 'date' command to display week number of the year?
There are two kinds of week:
Monday as first day
Sunday as first day
So its not all the same when you are in different day of a week(CentOS7):
[[email protected]:~]# date
Mon Aug 26 18:02:47 CST ...
- 301
15
votes
In `sed` how can I put one "&" between characters in a string?
Unix utilities:
fold -w1|paste -sd\& -
Explained:
"fold -w1" - will wrap an each input character to its own line
fold - wrap each input line to fit in specified width
-w, --width=WIDTH ...
- 3,972
15
votes
Accepted
bash - True if the length of string is (non)zero?
[ .. ] follows the same rules as all other commands, namely Word Splitting applies. If OUT is empty (or unset), $OUT will expand to nothing, not even an empty argument.
So, [ -n $OUT ] expands to [, -...
- 148k
14
votes
How to generate a random string?
I use:
base64 < /dev/urandom | tr -d 'O0Il1+/' | head -c 44; printf '\n'
This gives me 57 possible characters. The string can be copy-pasted (removed + and /) or printed and retyped as the ...
- 369
13
votes
Accepted
Print a string if the last character is odd
Short awk command:
awk '/[13579]$/' file > hostnames_odd.txt
[13579] - character class representing the list of allowed digits (odd numbers)
$ - the end of the string/line
Result:
$ cat ...
- 30.8k
13
votes
Accepted
Why grep does not work properly in this case?
.... what am I doing wrong? ... apart from making it far more complicated that it needs to be ... you are trying to match multiple spaces and leading spaces that are not in your strings in all of the ...
- 4,851
12
votes
Accepted
completely ignore lines that start with a specific pattern
Without using next, using negation instead.
Input:
$ cat f.txt
GOOD STUFF
----------------
IRRELEVENT DATA
----------------
IGNORE ALL THESE
----------------
END OF IT
----------------
GOOD STUFF
I ...
- 1,123
12
votes
Accepted
Test if variable contains newline (POSIX)
You can put a hard newline in a variable, and pattern match with case.
$ cat nl.sh
#!/bin/sh
nl='
'
case "$1" in
*$nl*) echo has newline ;;
*) echo no newline ;;
esac
$ dash nl.sh $'...
- 148k
12
votes
Accepted
How to POSIX-ly count the number of lines in a string variable?
The simple answer is that wc -l <<< "${string_variable}" is a ksh/bash/zsh shortcut for printf "%s\n" "${string_variable}" | wc -l.
There are actually differences in the way <<< and ...
12
votes
Accepted
How to treat a command output as text
You are attempting to execute the contents of $pvs_var as a command, rather than passing the string to awk.
To fix this, add an echo or printf in your if statement:
if [[ $(echo "$pvs_var" | awk '{ ...
- 4,286
12
votes
Accepted
Convert array type variable to string type with space delimiter
If the first character of IFS variable is a space (which it is by default), you can use the star index in double quotes.
#! /bin/bash
arr[0]=2019-06-26
arr[1]=15:21:54
string="${arr[*]}"
printf "'%s'...
- 49.4k
12
votes
Accepted
Command to keep only a portion of JSON data from each line?
With a proper jq tool:
jq -c '{"b": .b}' test.json
The output:
{"b":"sd"}
{"b":"bfgg"}
- 30.8k
Only top scored, non community-wiki answers of a minimum length are eligible