A shell pipe is a way to connect the output of one program to the input of another program without any temporary file. The syntax is:[donotprint][/donotprint]
command1 | command2 | commandN
OR
command1 | filter_data_command > output
OR
get_data_command | verify_data_command | process_data_command | format_data_command > output.data.file
How to use pipes to connect programs
Use the vertical bar (|) between two commands. In this example, send netstat command output to grep command i.e. find out if nginx process exits or not in the system:
# netstat -tulpn | grep nginx
Sample outputs:
How to get exit status of process that’s piped to another
The syntax is as follows to get exit codes of all piped commands:
command1 | command2 echo "${PIPESTATUS[@]}" |
OR
command1 | command2 echo "${PIPESTATUS[0]} ${PIPESTATUS[1]}" |
PIPESTATUS is an array variable containing a list of exit status values from the processes in the most-recently-executed foreground pipeline. Try the following commands:
netstat -tulpn | grep nginx echo "${PIPESTATUS[@]}" true | true echo "The exit status of first command ${PIPESTATUS[0]}, and the second command ${PIPESTATUS[1]}" true | false echo "The exit status of first command ${PIPESTATUS[0]}, and the second command ${PIPESTATUS[1]}" false | false | true echo "The exit status of first command ${PIPESTATUS[0]}, second command ${PIPESTATUS[1]}, and third command ${PIPESTATUS[2]}" |
Sample outputs:
Putting it all together
Here is a sample script that use ${PIPESTATUS[0]} to find out the exit status of mysqldump command in order to notify user on screen about database backup status:
#!/bin/bash ### Purpose: mysql.backup.sh : Backup database ### ### Author: Vivek Gite <https://www.cyberciti.biz>, under GPL v2.x+ or above. ### ### Change as per your needs ### set -eo pipefail MUSER='USERNAME-here' MPASS='PASSWORD-here' MHOST='10.0.3.100' DEST="/nfs42/backups/mysql" NOWFORMAT="%m_%d_%Y_%H_%M_%S%P" MYSQL="/usr/bin/mysql" MYSQLDUMP="/usr/bin/mysqldump" MKDIR="/bin/mkdir" RM="/bin/rm" GZIP="/bin/gzip" DATE="/bin/date" SED="/bin/sed" # Failsafe? Create dir # [ ! -d "$DEST" ] && $MKDIR -p "$DEST" # Filter db names DBS="$($MYSQL -u $MUSER -h $MHOST -p$MPASS -Bse 'show databases')" DBS="$($SED -e 's/performance_schema//' -e 's/information_schema//' <<<$DBS)" # Okay, let us go for db in $DBS do tTime=$(date +"${NOWFORMAT}") FILE="$DEST/${db}.${tTime}.gz" $MYSQLDUMP -u $MUSER -h $MHOST -p$MPASS $db | $GZIP -9 > $FILE if [ ${PIPESTATUS[0]} -ne "0" ]; then echo "The command $MYSQLDUMP failed with error code ${PIPESTATUS[0]}." exit 1 else echo "Database $db dump successfully." fi done |
A note about zsh user
Use the array called pipestatus as follows:
true | true echo "${pipestatus[1]} ${pipestatus[2]}" |
Outputs:
0 0
Conclusion – Finding exit codes of all piped commands
The exit status of a pipeline is the exit status of the last command in the pipeline, unless the pipefail option is enabled too. Just add the following at the top of your bash shell script:
set -eo pipefail
To pipe output and capture exit status in Bash shell use:
date | grep 2018 echo "${PIPESTATUS[0]} ${PIPESTATUS[1]}" false | true echo "${PIPESTATUS[0]} ${PIPESTATUS[1]}" |
For more info see:
- The exit status of a command
- Bourne Shell Exit Status Examples
- GNU/bash man page online here
- The bash man page here
4 comment
Very useful, I use pipe commands a lot.
I am learning every day.
Thanks for sharing.
I am a Fedora user and I installed zsh and enabled it, but at the first reboot, I got a menu of configurations options of zsh and I don’t know how to configure code completion and other configuration options. Do you can help me?
Very useful, thanks! (especially by the zsh tip).