Unix shells offer a type of loop that takes an enumerated list of string values, rather than counting through a sequence of numbers or looping while some input condition is true. This makes shell scripts very convenient for working with sets of files or arbitrary sets of values. This type of loop is well suited for use with globbing (file name patterns using wild cards, as discussed in the section called “Globbing (File Specifications)”):

#!/bin/sh -e

# Process input-1.txt, input-2.txt, etc.
for file in input-*.txt
do
    ./myprog $file
done

#!/bin/csh -ef

# Process input-1.txt, input-2.txt, etc.
foreach file (input-*.txt)
    ./myprog $file
end

These loops are not limited to using file names, however. We can use them to iterate through any list of string values:

#!/bin/sh -e

for fish in flounder gobie hammerhead manta moray sculpin
do
    printf "%s\n" $fish
done

#!/bin/sh -e

for c in 1 2 3 4 5 6 7 8 9 10
do
    printf "%d\n" $c
done

To iterate through a list of integers too long to type out, we can utilize the seq command, which takes a starting value, optionally an increment value, and an ending value, and prints the sequence to the standard output. We can use shell output capture (the section called “Output Capture”) to represent the output of the seq command as a string in the script:

#!/bin/sh -e

# Count from 0 to 1000 in increments of 5
for c in $(seq 0 5 1000); do
    printf "%d\n" $c
done

#!/bin/csh -ef

foreach c (`seq 0 5 1000`)
    printf "%s\n" $c
end

The seq can even be used to embed integer values in a non-integer list:

#!/bin/sh -e

# Process all human chromosomes
for chromosome in $(seq 1 22) X Y; do
    printf "chr%s\n" $chromosome
done

#!/bin/sh -e

# Download genome data from the ACME genome project
site=http://server.with.my.files/directory/with/my/files
for file in frog1 frog2 frog3 toad1 toad2 toad3; do
    printf "Fetching $site/$file.fasta.gz...\n"
    
    # Use filename from remote site and try to resume interrupted
    # transfers if a partial download already exists
    curl --continue-at - --remote-name $site/$file.fasta.gz
fi

#!/bin/csh -ef

# Download genome data from the ACME genome project
set site=http://server.with.my.files/directory/with/my/files
foreach file (frog1 frog2 frog3 toad1 toad2 toad3)
    printf "Fetching $site/$file.fasta.gz...\n"
    
    # Use filename from remote site and try to resume interrupted
    # transfers if a partial download already exists
    curl --continue-at - --remote-name $site/$file.fasta.gz
end

A for or foreach loop is only convenient for iterating through a fixed set of values or a sequence generated by a program such as seq. Sometimes we may need to terminate a loop based on inputs that are unknown when the loop begins, or values computed over the course of the loop.

The while loop is a more general loop that iterates as long as some condition is true. It uses the same types of expressions as an if command. The while loop can be used to iterate through long integer sequences, as we might do with seq and a for/foreach loop:

#!/bin/sh -e

c=1
while [ $c -le 100 ]
do
    printf "%d\n" $c
    c=$(($c + 1))        # (( )) encloses an integer expression
done

Note again that the [ above is an external command, as discussed in the section called “Command Exit Status”, so we must use white space to separate the arguments.

#!/bin/csh -ef

set c = 1
while ( $c <= 100 )
    printf "%d\n" $c
    @ c = $c + 1        # @ is like set, but indicates an integer expression
end

While loops can also be used to iterate until an input condition is met:

#!/bin/sh -e

continue=''
while [ 0"$continue" != 0'y' ] && [ 0"$continue" != 0'n' ]; do
    printf "Would you like to continue? (y/n) "
    read continue
done

#!/bin/csh -ef

set continue=''
while ( ("$continue" != 'y') && ("$continue" != 'n') )
    printf "Continue? (y/n) "
    set continue="$<"
end

We may even want a loop to iterate forever. This is often useful when using a computer to collect data at regular intervals. It is up to the user to terminate the process using Ctrl+c or kill.

#!/bin/sh -e

# 'true' is an external command that always returns an exit status of 0
while true; do
    sample-data     # Read instrument
    sleep 10        # Pause for 10 seconds without using any CPU time
done

#!/bin/csh -ef

while ( 1 )
    sample-data     # Read instrument
    sleep 10        # Pause for 10 seconds without using any CPU time
end