Editing Job Control Language (section)

===Concatenating input files===
OS JCL allows programmers to concatenate ("chain") input files so that they appear to the program as ''one'' file, for example
<syntaxhighlight lang="jcl">

//INPUT01 DD DSN=MYFILE01,DISP=SHR
//        DD DSN=JOESFILE,DISP=SHR
//        DD DSN=SUESFILE,DISP=SHR
</syntaxhighlight>

The 2nd and third statements have no value in the name field, so OS treats them as concatenations. The files must be of the same basic type (almost always sequential), and must have the same record length, however the block length need not be the same.

In early versions of the OS (certainly before OS/360 R21.8) the block length must be in decreasing order, or the user must inspect each instance and append to the named DD statement the maximum block length found, as in, for example,
<syntaxhighlight lang="jcl">

//INPUT01 DD DSN=MYFILE01,DISP=SHR,BLKSIZE=800
//        DD DSN=JOESFILE,DISP=SHR (BLKSIZE assumed to be equal to or less than 800)
//        DD DSN=SUESFILE,DISP=SHR (BLKSIZE assumed to be equal to or less than 800)
</syntaxhighlight>

In later versions of the OS (certainly after OS/MVS R3.7 with the appropriate "selectable units") the OS itself, during allocation, would inspect each instance in a concatenation and would substitute the maximum block length which was found.

A usual fallback was to simply determine the maximum possible block length on the device, and specify that on the named DD statement, as in, for example,
<syntaxhighlight lang="jcl">

//INPUT01 DD DSN=MYFILE01,DISP=SHR,BLKSIZE=8000
//        DD DSN=JOESFILE,DISP=SHR (BLKSIZE assumed to be equal to or less than 8000)
//        DD DSN=SUESFILE,DISP=SHR (BLKSIZE assumed to be equal to or less than 8000)
</syntaxhighlight>

The purpose of this fallback was to ensure that the access method would allocate an input buffer set which was large enough to accommodate any and all of the specified datasets.