I have a bash script that produces some text from a pipe of commands. Based on a command line option I want to do some validation on the output. For a contrived example...
CHECK_OUTPUT=$1
...
check_output()
{
if [[ "$CHECK_OUTPUT" != "--check" ]]; then
# Don't check the output. Passthrough and return.
cat
return 0
fi
# Check each line exists in the fs root
while read line; do
if [[ ! -e "/$line" ]]; then
echo "Error: /$line does not exist"
return 1
fi
echo "$line"
done
return 0
}
ls /usr | grep '^b' | check_output
[EDIT] better example: https://stackoverflow.com/a/52539364/1888983
This is really useful, particularly if I have multiple functions that can becomes passthroughs. Yes, I could move the CHECK_OUTPUT conditional and create a pipe with or without check_output but I'd need to write lines for each combination for more functions. If there are better ways to dynamically build a pipe I'd like to know.
The problem is the "useless use of cat". Can this be avoided and make check_output like it wasn't in the pipe at all?
Yes, you can do this -- by making your function a wrapper that conditionally injects a pipeline element, instead of being an unconditional pipeline element itself. For example:
maybe_checked() {
if [[ $CHECK_OUTPUT != "--check" ]]; then
"$#" # just run our arguments as a command, as if we weren't here
else
# run our arguments in a process substitution, reading from stdout of same.
# ...some changes from the original code:
# IFS= stops leading or trailing whitespace from being stripped
# read -r prevents backslashes from being processed
local line # avoid modifying $line outside our function
while IFS= read -r line; do
[[ -e "/$line" ]] || { echo "Error: /$line does not exist" >&2; return 1; }
printf '%s\n' "$line" # see https://unix.stackexchange.com/questions/65803
done < <("$#")
fi
}
ls /usr | maybe_checked grep '^b'
Caveat of the above code: if the pipefail option is set, you'll want to check the exit status of the process substitution to have complete parity with the behavior that would otherwise be the case. In bash version 4.3 or later (IIRC), $? is modified by process substitutions to have the relevant PID, which can be waited for to retrieve exit status.
That said, this is also a use case wherein using cat is acceptable, and I'm saying this as a card-carying member of the UUOC crowd. :)
Adopting the examples from John Kugelman's answers on the linked question:
maybe_sort() {
if (( sort )); then
"$#" | sort
else
"$#"
fi
}
maybe_limit() {
if [[ -n $limit ]]; then
"$#" | head -n "$limit"
else
"$#"
fi
}
printf '%s\n' "${haikus[#]}" | maybe_limit maybe_sort sed -e 's/^[ \t]*//'
Related
In the following program, if I set the variable $foo to the value 1 inside the first if statement, it works in the sense that its value is remembered after the if statement. However, when I set the same variable to the value 2 inside an if which is inside a while statement, it's forgotten after the while loop. It's behaving like I'm using some sort of copy of the variable $foo inside the while loop and I am modifying only that particular copy. Here's a complete test program:
#!/bin/bash
set -e
set -u
foo=0
bar="hello"
if [[ "$bar" == "hello" ]]
then
foo=1
echo "Setting \$foo to 1: $foo"
fi
echo "Variable \$foo after if statement: $foo"
lines="first line\nsecond line\nthird line"
echo -e $lines | while read line
do
if [[ "$line" == "second line" ]]
then
foo=2
echo "Variable \$foo updated to $foo inside if inside while loop"
fi
echo "Value of \$foo in while loop body: $foo"
done
echo "Variable \$foo after while loop: $foo"
# Output:
# $ ./testbash.sh
# Setting $foo to 1: 1
# Variable $foo after if statement: 1
# Value of $foo in while loop body: 1
# Variable $foo updated to 2 inside if inside while loop
# Value of $foo in while loop body: 2
# Value of $foo in while loop body: 2
# Variable $foo after while loop: 1
# bash --version
# GNU bash, version 4.1.10(4)-release (i686-pc-cygwin)
echo -e $lines | while read line
...
done
The while loop is executed in a subshell. So any changes you do to the variable will not be available once the subshell exits.
Instead you can use a here string to re-write the while loop to be in the main shell process; only echo -e $lines will run in a subshell:
while read line
do
if [[ "$line" == "second line" ]]
then
foo=2
echo "Variable \$foo updated to $foo inside if inside while loop"
fi
echo "Value of \$foo in while loop body: $foo"
done <<< "$(echo -e "$lines")"
You can get rid of the rather ugly echo in the here-string above by expanding the backslash sequences immediately when assigning lines. The $'...' form of quoting can be used there:
lines=$'first line\nsecond line\nthird line'
while read line; do
...
done <<< "$lines"
UPDATED#2
Explanation is in Blue Moons's answer.
Alternative solutions:
Eliminate echo
while read line; do
...
done <<EOT
first line
second line
third line
EOT
Add the echo inside the here-is-the-document
while read line; do
...
done <<EOT
$(echo -e $lines)
EOT
Run echo in background:
coproc echo -e $lines
while read -u ${COPROC[0]} line; do
...
done
Redirect to a file handle explicitly (Mind the space in < <!):
exec 3< <(echo -e $lines)
while read -u 3 line; do
...
done
Or just redirect to the stdin:
while read line; do
...
done < <(echo -e $lines)
And one for chepner (eliminating echo):
arr=("first line" "second line" "third line");
for((i=0;i<${#arr[*]};++i)) { line=${arr[i]};
...
}
Variable $lines can be converted to an array without starting a new sub-shell. The characters \ and n has to be converted to some character (e.g. a real new line character) and use the IFS (Internal Field Separator) variable to split the string into array elements. This can be done like:
lines="first line\nsecond line\nthird line"
echo "$lines"
OIFS="$IFS"
IFS=$'\n' arr=(${lines//\\n/$'\n'}) # Conversion
IFS="$OIFS"
echo "${arr[#]}", Length: ${#arr[*]}
set|grep ^arr
Result is
first line\nsecond line\nthird line
first line second line third line, Length: 3
arr=([0]="first line" [1]="second line" [2]="third line")
You are asking this bash FAQ. The answer also describes the general case of variables set in subshells created by pipes:
E4) If I pipe the output of a command into read variable, why
doesn't the output show up in $variable when the read command finishes?
This has to do with the parent-child relationship between Unix
processes. It affects all commands run in pipelines, not just
simple calls to read. For example, piping a command's output
into a while loop that repeatedly calls read will result in
the same behavior.
Each element of a pipeline, even a builtin or shell function,
runs in a separate process, a child of the shell running the
pipeline. A subprocess cannot affect its parent's environment.
When the read command sets the variable to the input, that
variable is set only in the subshell, not the parent shell. When
the subshell exits, the value of the variable is lost.
Many pipelines that end with read variable can be converted
into command substitutions, which will capture the output of
a specified command. The output can then be assigned to a
variable:
grep ^gnu /usr/lib/news/active | wc -l | read ngroup
can be converted into
ngroup=$(grep ^gnu /usr/lib/news/active | wc -l)
This does not, unfortunately, work to split the text among
multiple variables, as read does when given multiple variable
arguments. If you need to do this, you can either use the
command substitution above to read the output into a variable
and chop up the variable using the bash pattern removal
expansion operators or use some variant of the following
approach.
Say /usr/local/bin/ipaddr is the following shell script:
#! /bin/sh
host `hostname` | awk '/address/ {print $NF}'
Instead of using
/usr/local/bin/ipaddr | read A B C D
to break the local machine's IP address into separate octets, use
OIFS="$IFS"
IFS=.
set -- $(/usr/local/bin/ipaddr)
IFS="$OIFS"
A="$1" B="$2" C="$3" D="$4"
Beware, however, that this will change the shell's positional
parameters. If you need them, you should save them before doing
this.
This is the general approach -- in most cases you will not need to
set $IFS to a different value.
Some other user-supplied alternatives include:
read A B C D << HERE
$(IFS=.; echo $(/usr/local/bin/ipaddr))
HERE
and, where process substitution is available,
read A B C D < <(IFS=.; echo $(/usr/local/bin/ipaddr))
Hmmm... I would almost swear that this worked for the original Bourne shell, but don't have access to a running copy just now to check.
There is, however, a very trivial workaround to the problem.
Change the first line of the script from:
#!/bin/bash
to
#!/bin/ksh
Et voila! A read at the end of a pipeline works just fine, assuming you have the Korn shell installed.
This is an interesting question and touches on a very basic concept in Bourne shell and subshell. Here I provide a solution that is different from the previous solutions by doing some kind of filtering. I will give an example that may be useful in real life. This is a fragment for checking that downloaded files conform to a known checksum. The checksum file look like the following (Showing just 3 lines):
49174 36326 dna_align_feature.txt.gz
54757 1 dna.txt.gz
55409 9971 exon_transcript.txt.gz
The shell script:
#!/bin/sh
.....
failcnt=0 # this variable is only valid in the parent shell
#variable xx captures all the outputs from the while loop
xx=$(cat ${checkfile} | while read -r line; do
num1=$(echo $line | awk '{print $1}')
num2=$(echo $line | awk '{print $2}')
fname=$(echo $line | awk '{print $3}')
if [ -f "$fname" ]; then
res=$(sum $fname)
filegood=$(sum $fname | awk -v na=$num1 -v nb=$num2 -v fn=$fname '{ if (na == $1 && nb == $2) { print "TRUE"; } else { print "FALSE"; }}')
if [ "$filegood" = "FALSE" ]; then
failcnt=$(expr $failcnt + 1) # only in subshell
echo "$fname BAD $failcnt"
fi
fi
done | tail -1) # I am only interested in the final result
# you can capture a whole bunch of texts and do further filtering
failcnt=${xx#* BAD } # I am only interested in the number
# this variable is in the parent shell
echo failcnt $failcnt
if [ $failcnt -gt 0 ]; then
echo $failcnt files failed
else
echo download successful
fi
The parent and subshell communicate through the echo command. You can pick some easy to parse text for the parent shell. This method does not break your normal way of thinking, just that you have to do some post processing. You can use grep, sed, awk, and more for doing so.
I use stderr to store within a loop, and read from it outside.
Here var i is initially set and read inside the loop as 1.
# reading lines of content from 2 files concatenated
# inside loop: write value of var i to stderr (before iteration)
# outside: read var i from stderr, has last iterative value
f=/tmp/file1
g=/tmp/file2
i=1
cat $f $g | \
while read -r s;
do
echo $s > /dev/null; # some work
echo $i > 2
let i++
done;
read -r i < 2
echo $i
Or use the heredoc method to reduce the amount of code in a subshell.
Note the iterative i value can be read outside the while loop.
i=1
while read -r s;
do
echo $s > /dev/null
let i++
done <<EOT
$(cat $f $g)
EOT
let i--
echo $i
How about a very simple method
+call your while loop in a function
- set your value inside (nonsense, but shows the example)
- return your value inside
+capture your value outside
+set outside
+display outside
#!/bin/bash
# set -e
# set -u
# No idea why you need this, not using here
foo=0
bar="hello"
if [[ "$bar" == "hello" ]]
then
foo=1
echo "Setting \$foo to $foo"
fi
echo "Variable \$foo after if statement: $foo"
lines="first line\nsecond line\nthird line"
function my_while_loop
{
echo -e $lines | while read line
do
if [[ "$line" == "second line" ]]
then
foo=2; return 2;
echo "Variable \$foo updated to $foo inside if inside while loop"
fi
echo -e $lines | while read line
do
if [[ "$line" == "second line" ]]
then
foo=2;
echo "Variable \$foo updated to $foo inside if inside while loop"
return 2;
fi
# Code below won't be executed since we returned from function in 'if' statement
# We aready reported the $foo var beint set to 2 anyway
echo "Value of \$foo in while loop body: $foo"
done
}
my_while_loop; foo="$?"
echo "Variable \$foo after while loop: $foo"
Output:
Setting $foo 1
Variable $foo after if statement: 1
Value of $foo in while loop body: 1
Variable $foo after while loop: 2
bash --version
GNU bash, version 3.2.51(1)-release (x86_64-apple-darwin13)
Copyright (C) 2007 Free Software Foundation, Inc.
Though this is an old question and asked several times, here's what I'm doing after hours fidgeting with here strings, and the only option that worked for me is to store the value in a file during while loop sub-shells and then retrieve it. Simple.
Use echo statement to store and cat statement to retrieve. And the bash user must chown the directory or have read-write chmod access.
#write to file
echo "1" > foo.txt
while condition; do
if (condition); then
#write again to file
echo "2" > foo.txt
fi
done
#read from file
echo "Value of \$foo in while loop body: $(cat foo.txt)"
I'm writing a POSIX compliant script in dash so I am having to get creative with using fake arrays.
Contents of fake_array.sh
fake_array_job() {
array="$1"
job_name="$2"
comma_count="$(echo "$array" | grep -o -F ',' | wc -l)"
if [ "$comma_count" -lt '1' ]; then
echo 'You gave a fake array to fake_array_job that does not contain at least one comma. Exiting...'
exit
fi
array_count="$(( comma_count + 1 ))"
position=1
while [ "$position" -le "$array_count" ]; do
item="$(echo "$array" | cut -d ',' -f "$position")"
"$job_name" || exit
position="$(( position + 1 ))"
done
}
Contents of script.sh
#!/bin/sh
. fake_array.sh
job_to_do() {
echo "$item"
}
fake_array_job 'goat,pig,sheep' 'job_to_do'
second_job() {
echo "$item"
}
fake_array_job 'apple,orange' 'second_job'
I am aware that it may seem silly to use a unique name for each job I pass to fake_array_job, but I like that I have to type it twice because it helps to reduce human error.
I keep reading that it is a bad idea to use a variable as a command. Does my use of "$job_name" to run a function have any negative implications as it concerns stability, security or efficiency?
(Read to the end for a good suggestion by Charles Duffy. I'm too lazy to completely rewrite my answer to mention it earlier...)
You can iterate over the "array" using simple parameter expansions without requiring multiple elements in the array.
fake_array_job() {
args=${1%,}, # Ensure the array ends with a comma
job_name=$2
while [ -n "$args" ]; do
item=${args%%,*}
"$job_name" || exit
args=${args#*,}
done
}
One problem with the above is that assures that the array is comma-terminated by assuming that foo,bar, is not a comma-delimited array with an empty last element. A better (though uglier) solution is to use read to break up the array.
fake_array_job () {
args=$1
job_name=$2
rest=$args
while [ -n "$rest" ]; do
IFS=, read -r item rest <<EOF
$rest
EOF
"$job_name" || exit
done
}
(You can use <<-EOF and make sure the here doc is indented with tabs, but it's hard to convey that here, so I'll just leave the ugly version.)
There's also Charles Duffy's good suggestion of using case to pattern match on the array to see if there are any commas left or not:
while [ -n "$args" ]; do
case $var in
*,*) next=${args%%,*}; var=${args#*,}; "$cmd" "$next";;
*) "$cmd" "$var"; break;;
esac;
done
I am learning bash. I would like to get the return value and matched line by grep at once.
if cat 'file' | grep 'match_word'; then
match_by_grep="$(cat 'file' | grep 'match_word')"
read a b <<< "${match_by_grep}"
fi
In the code above, I used grep twice. I cannot think of how to do it by grep once. I am not sure match_by_grep is always empty even when there is no matched words because cat may output error message.
match_by_grep="$(cat 'file' | grep 'match_word')"
if [[ -n ${match_by_grep} ]]; then
# match_by_grep may be an error message by cat.
# So following a and b may have wrong value.
read a b <<< "${match_by_grep}"
fi
Please tell me how to do it. Thank you very much.
You can avoid the double use of grep by storing the search output in a variable and seeing if it is not empty.
Your version of the script without double grep.
#!/bin/bash
grepOutput="$(grep 'match_word' file)"
if [ ! -z "$grepOutput" ]; then
read a b <<< "${grepOutput}"
fi
An optimization over the above script ( you can remove the temporary variable too)
#!/bin/bash
grepOutput="$(grep 'match_word' file)"
[[ ! -z "$grepOutput" ]] && (read a b <<< "${grepOutput}")
Using double-grep once for checking if-condition and once to parse the search result would be something like:-
#!/bin/bash
if grep -q 'match_word' file; then
grepOutput="$(grep 'match_word' file)"
read a b <<< "${grepOutput}"
fi
When assigning a variable with a string containing a command expansion, the return code is that of the (rightmost) command being expanded.
In other words, you can just use the assignment as the condition:
if grepOutput="$(cat 'file' | grep 'match_word')"
then
echo "There was a match"
read -r a b <<< "${grepOutput}"
(etc)
else
echo "No match"
fi
Is this what you want to achieve?
grep 'match_word' file ; echo $?
$? has a return value of the command run immediately before.
If you would like to keep track of the return value, it will be also useful to have PS1 set up with $?.
Ref: Bash Prompt with Last Exit Code
I have a series of commands chained together with pipes:
should_create_one_line | expects_one_line
The first command should_create_one_line should produce an output that only has one line, but under strange circumstances it is possible for the output to be multiline or empty.
I would like to add a step in between these two, validate_one_line:
should_create_one_line | validate_one_line | expects_one_line
If its input contains exactly 1 line then validate_one_line will simply output its input. If its input contains more than 1 line or is empty then validate_one_line should cause the whole sequence of steps to stop and return an error code.
What command can I use for validate_one_line?
Use read. Here's a shell function that meets your specs:
exactly_one_line() {
local line # Use to echo the line
read -r line || return # Guarantee at least one line is read
read && return 1 # Indicate failure if another line is successfully read
echo "$line"
}
Notes
"One line" assumes a single line followed by a newline. If your input could be like, a file with contents but no newlines, then this will fail.
Given a pipeline like a|b, a cannot prevent b from running. At a minimum, b needs to handle when a produces no output.
Demo:
$ wc -l empty oneline twolines
0 empty
1 oneline
2 twolines
3 total
$ exactly_one_line < empty; echo $?
1
$ exactly_one_line < oneline; echo $?
oneline
0
$ exactly_one_line < twolines; echo $?
1
First off, you should seriously consider adding the validation code to expects_one_line. According to this post, each process starts in its own subshell, meaning that even if validate_one_line fails, you will get an error in expects_one_line because it will try to run with no input (or a blank line). That being said, here is a bash one-liner that you can insert into your pipe to validate:
should_create_one_line.sh | ( var="$(cat)"; [ $(echo "$var" | wc -l) -ne 1 ] && exit 1 || echo "$var") | expects_one_line.sh
The problem here is that when the validation subshell returns in the exit 1 case, expects_one_line.sh will still get a single blank line. If this works for you, then great. If not, it would be better to just put the following into the beginning of expects_one_line.sh:
input="$(cat)"
[ $(echo "$var" | wc -l) -ne 1 ] && exit 1
This would guarantee that expects_one_line.sh fails properly when getting a single line without having to wonder about what the empty line that the validation outputs will do to the script.
You may find this post helpful: How to read mutliline input from stdin into variable and how to print one out in shell(sh,bash)?
You can use a bash script to check the incoming data and call the other command when the input is only 1 line
The following code starts cat when it is ONLY fet in 1 line
sh -c 'while read CMD; do [ ! -z "$LINE" ] && exit 1; LINE=$CMD; done; [ -z "$LINE" ] && exit 1; printf "%s\n" $LINE | "$0" "$#"' cat
How this works
Try reading a line, if failed go to step 5
If variable $LINE is NOT empty, goto step 6
Save line inside variable $LINE
Goto step 1
If $LINE is NOT empty, goto step 7
Exit the program with status code 1
Call our program and pass our $line to it using printf
Example usage:
Printing out only if grep found 1 match:
grep .... | sh -c 'while read CMD; do [ ! -z "$LINE" ] && exit 1; LINE=$CMD; done; [ -z "$LINE" ] && exit 1; printf "%s\n" $LINE | "$0" "$#"' cat
Example of the question poster:
should_create_one_line | sh -c 'while read CMD; do [ ! -z "$LINE" ] && exit 1; LINE=$CMD; done; [ -z "$LINE" ] && exit 1; printf "%s\n" $LINE | "$0" "$#"' expects_one_line
I try to generate a csv file through a lot of functions like that :
function get_sudo_version {
sudo -V 2>/dev/null|grep -i "sudo version"
}
sudo_version=$(get_sudo_version)
Function above is a simple example but in some cases i cannot be sure the output is correct.
I would like to know what is the best way to validate the function return one text line only.
I thought about something like that
function validate_output {
output=$1;
echo $1|grep -q "\n";
echo $?;
}
mytest="val1
err2
err3"
But it's obviously not working because the variable does not keep the retrun line character:
echo $mytest
val1 err2 err3
So if someone has a good idea of how i could wirte a generic check function i would be glad.
Thanks
If you have GNU grep, you could simply ensure that grep doesn't produce more than one line of output in the first place via grep -m 1. Alternatively, use sed '/sudo version/!d;q' instead of grep.
A function that simply checks lines of input while passing them through might look like:
shopt -s lastpipe # if bash
# ksh or bash
validate_output() {
(( $(tee >(wc -l) >&2) == 1 ))
} 2>&1
# bash
validate_output2() {
local lines
tee /dev/fd/2 | mapfile -tc1 -C '((++lines < 2)) || return; :'
((lines))
} 2>&1
get_sudo_version | validate_output || echo failed
Many variations on that possible of course. IMO it's pretty pointless and shouldn't be used for something like this. Just design your get_sudo_version so that it guarantees the right results.
You can count lines with wc -l:
$ LINES="$(echo -n | wc -l)"; [[ "$LINES" -gt 0 ]] && echo 'at least one line'
$ LINES="$(echo -e 'a\nb\nc' | wc -l)"; [[ "$LINES" -gt 0 ]] && echo 'at least one line'
at least one line